Gentle knock (3)
I believe the answer is:
tap
'gentle knock' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tap that I've seen before include "It's called a faucet in America" , "that's less hard than 11 [BASH] or 22 [BATTER]" , "Light blow; draw sap from (tree)" , "non-violent strike" , "Dancing" .)