Cutting tart (9)
I believe the answer is:
trenchant
I believe this is a double definition.
'cutting' is the first definition.
(I've seen this before)
'tart' is the second definition.
(thesaurus)
(Other definitions for trenchant that I've seen before include "blunt" , "caustic?" , "Forceful" , "Fierce" , "Vigorous, incisive" .)