Knock at nail (6)
I believe the answer is:
attack
'knock' is the definition.
(I know this)
'at nail' is the wordplay.
'nail' becomes 'tack' (tack is a kind of nail).
'at'+'tack'='ATTACK'
(Other definitions for attack that I've seen before include "act of aggression" , "Ambush - mug - criticise - fit" , "storm" , "Act violently against or criticise strongly" , "Strike" .)