Backing screw for round handle (4)
I believe the answer is:
knob
'round handle' is the definition.
(I've seen this before)
'backing screw' is the wordplay.
'backing' says the letters should be written backwards.
'screw' becomes 'bonk' (synonyms).
'bonk' in reverse letter order is 'KNOB'.
'for' is the link.
(Other definitions for knob that I've seen before include "Handle or dial" , "Rounded handle" , "Hump" , "Turn it to open door" , "Protuberance" .)