It fits over the centre of a wheel (6)
I believe the answer is:
hubcap
'it fits over the centre of a wheel' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hubcap that I've seen before include "It fits over the centre of a wheel" , "Metal cover on wheel" , "Cover on a car wheel" , "Vehicle wheel trim" , "Wheel attachment" .)