Passage in Question:

Three years ago, George—let us start there—soon after you left the Circus—your successor Saul Enderby—your worthy successor—under pressure from a concerned Cabinet —by concerned I mean newly formed—decided on certain far-reaching changes of intelligence practice.

In short, there are two possibilities. Briefly:

"Concerned" means: a cabinet that is concerned OR

"Concerned" means: a cabinet that is concerning (to the speaker)**

Here is possibility #1 first: "concerned" here may be synonymous with problematic--based on the context outside of looking to a dictionary on standard modern English. This would be a non-standard usage of the word "concerned".

Now, on to the second possibility: The second possibility requires the standard usage: "concerned" in the phrase "concerned cabinet" may simply mean that there is a Cabinet that demonstrates concern about some topic or practice.

Please read below for a deeper discussion. Here lies a literary analysis that is provided to obtain context for Le Carré's intended usage of the word "concerned":

Without familiarity with Le Carré's Smiley's People or Le Carré's style or time of writing (which may affect popular but non-standardized styles of writing),there are a few points I would address first.

The Context: The character's personality or feelings at the moment: nuanced, complex, choppy, and possibly erratic. Le Carré may mean to draw a character in our mind's canvas as a character whose speech is full of nuance, suggesting that the character's mind, while discussing or mentioning something, contains opinions that the character wishes to ascribe to that something. That something here is the "cabinet". The "cabinet" is the topic of concern. And, those concerns are met with opinions that are expressed in a disorganized manner, signified by the use of em-dashes which show an interruption in the flow of the speech. It may also mean to suggest a personality trait of the character, who may come off as very granular (detailed), and very elaborate--perhaps so elaborate that it would be considered obtuse (similar to how you may feel about me writing this). The character's flow of thoughts is also choppy, which may mean to reflect the character's current internal state of feelings. There is also humor riddled in his choppiness--almost a sarcasm at certain parts. Maybe, the character is nervous. Maybe, the character is chaotic. You may know better based on your reading of the book up to that passage. Taken together, long story short, the character in focused is hyper-detailed, a little chaotic and even possibly flustered. This affects how we may interpret the particular wording.

Now, with the above context, I think I can easily answer your question, which amounts to "What does "concerned" mean here (as it is used in the aforementioned passage in question)?"

So, now, the character first says "concerned cabinet" then, following an interjection of a nested thought that is signified by the em-dash, the character then states, "by concerned I mean newly formed", as if concerned was intended as a jeer/"dis" and he's rephrasing for sarcastic effect.

By "concerned" he means to say "Concerning"--i.e., there is a problematic situation arising. Since the word "concerned" is used as an adjective to modify "cabinet"--he means to say that the specific people who formed the cabinet are a cause for concern. The English is not modern or standard (at least according to my intuition of North American English). This makes sense because this book was written in 1979 but I would expect the language to be of very formal and educated characters--perhaps, even British characters if they are not American.

Thus, the use of the word "concerned" as a less-standard adjectival modifier when modifying the noun "cabinet" here is not intended to indicate that the cabinet is "feeling concerned"; Rather, the author intends that this character is (most likely) concerned about the Cabinet--at a minimum. He may further mean to suggest that the composition of the newly formed cabinet is concerning for more than just him. In other words, there were people put on the cabinet that may not be liked by the character or may be bad picks to be selected for the cabinet.