A gentle knocking in bathroom fitting (3)
I believe the answer is:
tap
This clue is a double definition.
'a gentle knocking' is the first definition.
(to tap is to touch or knock gently)
'bathroom fitting' is the second definition.
(item fitted in a bathroom)
'in' is the link.
(Other definitions for tap that I've seen before include "Thread cutter; strike" , "Light blow; draw sap from (tree)" , "British faucet" , "Intercept; light knock" , "Water controller" .)