Boss has mate round (4)
I believe the answer is:
knob
'boss' is the definition.
(I know that boss can be written as knob)
'mate round' is the wordplay.
'mate' becomes 'bonk' (bonking is a kind of mating).
'round' is a reversal indicator.
'bonk' backwards is 'KNOB'.
'has' is the link.
(Other definitions for knob that I've seen before include "Hump" , "hard swelling" , "Protuberance" , "Rounded handle" , "Round door-handle" .)