Caustic witticism by a man (7)
I believe the answer is:
pungent
'caustic' is the definition.
(I've seen this before)
'witticism by a man' is the wordplay.
'witticism' becomes 'pun' (pun is a kind of witticism).
'by' means one lot of letters go next to another.
'a man' becomes 'gent' (gent is a kind of man).
'pun'+'gent'='PUNGENT'
(Other definitions for pungent that I've seen before include "Witty" , "bitter" , "Acrid - painful" , "Sharp tasting/smelling" , "Smelly" .)