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 subsidiary indication.
'keep' becomes 'stock' (**).
'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 "Vehicle adapted for deliberate crashes", "Production-line vehicle modified for racing", "Modified motor for racing and ramming", "Production vehicle used in racing", "Vehicle used in racing" and "Vehicle adapted for bumping races".)