Sort of hammer sharp nail (4)
I believe the answer is:
claw
I believe this clue is a double definition.
'sort of hammer' is the first definition.
'sharp nail' is the second definition.
'claw' can be an answer for 'nail' (thesaurus). I also know that 'sharp' relates to this answer.
(Other definitions for claw that I've seen before include "Crab's pincer" , "Pointed nail on animal's foot" , "Lobster's grasping structure" , "Crab pincer" , "Lobster's pincer" .)