Knock at sharp nail (6)
I believe the answer is:
attack
'knock' is the definition.
(I know that attack is a type of knock)
'at sharp nail' is the wordplay.
'sharp nail' becomes 'tack' (a tack is a nail).
'at'+'tack'='ATTACK'
(Other definitions for attack that I've seen before include "Strike" , "Show aggression" , "Offensive action" , "Go on the offensive" , "Hit out" .)