Caustic remark (4)
I believe the answer is:
barb
'caustic remark' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for barb that I've seen before include "Thorn or spike" , "Insult" , "Kind of thorn or wounding remark" , "Hurtful witticism - on wire" , "Hook - gibe" .)