Cut knuckle near index finger exasperates initially (5)
I believe the answer is:
knife
'cut' is the definition.
(I've seen this in another clue)
'knuckle near index finger exasperates initially' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'knuckle near index finger exasperates' is 'KNIFE'.
(Other definitions for knife that I've seen before include "Piece of cutlery" , "Cutlery item" , "Table implement" , "Edge tool" , "Cutting blade" .)