Cover on a car wheel (6)
I believe the answer is:
hubcap
'cover on a car wheel' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hubcap that I've seen before include "Protective plate for car wheel" , "Cover for centre of a car's wheel" , "It fits over the centre of a wheel" , "Metal disc to protect outside of car wheel" , "Metal cover on wheel" .)