Caustic witticism, man! (7)
I believe the answer is:
pungent
'caustic' is the definition.
(I've seen this before)
'witticism man' is the wordplay.
'witticism' becomes 'pun' (pun is a kind of witticism).
'man' becomes 'gent' (gent is a kind of man).
'pun'+'gent'='PUNGENT'
(Other definitions for pungent that I've seen before include "Biting" , "Strong-smelling" , "bitter" , "Acrid; sarcastic (humour)" , "Acrid to smell" .)