Knocks on the sink? (4)
I believe the answer is:
taps
'knocks' is the definition.
('tap' can be a synonym of 'knock')
'on the sink?' is the wordplay.
'on' is a charade indicator (letters next to each other).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'sink?' becomes 'aps' (I can't justify this - if you can you should believe this answer much more).
't' put next to 'aps' is 'TAPS'.
Can you help me to learn more?
(Other definitions for taps that I've seen before include "Bugs (wire)" , "The sound of lights out" , "They're faucets in America" , "Military signal and faucets" , "Knocks lightly on the faucets" .)