Racer shares coach (5,3)
I believe the answer is:
'racer' is the definition.
(stock car is a kind of racer)
'shares coach' is the wordplay.
'shares' becomes 'stock'.
'coach' becomes 'car' (I've seen this in other clues).
(Other definitions for stock car that I've seen before include "Vehicle adapted for bumping races" , "Production-line vehicle modified for racing" , "production saloon" , "Production vehicle used in racing" , "customised vehicle" .)