Caustic word-play by a chap (7)
I believe the answer is:
pungent
'caustic' is the definition.
(I've seen this before)
'word-play by a chap' is the wordplay.
'word play' becomes 'pun' (I've seen this in another clue).
'by' is a charade indicator (letters next to each other).
'a chap' becomes 'gent' ('gent' can be a synonym of 'chap').
'pun'+'gent'='PUNGENT'
(Other definitions for pungent that I've seen before include "Having a strong smell" , "Offensive" , "Biting" , "Strong and sharp" , "delivering powerful smack" .)