But his vocabulary is not necessarily limited (3,2,3,5)
I believe the answer is:
man of few words
'but his' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'vocabulary is not necessarily limited' is the wordplay.
I cannot really see how this works, but
'vocabulary' could be 'words' (I've seen this in another clue) and 'words' is present in the answer.
'not' could be 'no' (I've seen this in other clues) and 'no' is found within the answer.
'limited' could be 'few' (I've seen this in another clue) and 'few' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?