Gently knocks over the kitchen sink! (4)
I believe the answer is:
taps
'gently knocks' is the definition.
'taps' can be an answer for 'knocks' ('tap' can be a synonym of 'knock'). I'm unsure of the 'gently' bit.
'over the kitchen sink' is the wordplay.
'over' says to put letters next to each other.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'kitchen sink' becomes 'aps' (I can't explain this - if you can you should give a lot more credence to this answer).
'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; valves" , "pierces" , "Knocks gently" , "Military signal and faucets" , "Military signal to turn the lights out" .)