Keep short jack, say, in vehicle (5,3)
I believe the answer is:
Here is my best explanation:
'vehicle' is the definition.
(stock car is a kind of vehicle)
'keep short jack say' is the wordplay.
'keep' becomes 'stock' (I've seen this before).
'short' means to remove the last letter.
'jack say' becomes 'card' (jack is a kind of card).
'card' with its last letter removed is 'car'.
'in' is the link.
(Other definitions for stock car that I've seen before include "customised vehicle", "Vehicle used in racing", "Production-line vehicle modified for racing", "Modified motor for racing and ramming", "Vehicle adapted for bumping races", "Type of racing", "Production vehicle used in racing", "production saloon" and "Vehicle adapted for deliberate crashes".)