Trenchant; acute (8)
I believe the answer is:
incisive
This clue is a double definition.
'trenchant' is the first definition.
(I've seen this in another clue)
'acute' is the second definition.
(I know that acute can be written as incisive)
(Other definitions for incisive that I've seen before include "Analytical, concise" , "Clear and probing, trenchant" , "Clear and cutting" , "Intelligently analytical and clear-thinking" , "Piercing" .)