Participating in next race — a bonus (5)
I believe the answer is:
extra
'bonus' is the definition.
(I've seen this before)
'participating in next race a' is the wordplay.
'participating in' says the answer is hidden in the clue.
'EXTRA' is hidden within 'next race a'.
(Other definitions for extra that I've seen before include "lesser player" , "outside" , "score (not a hit)" , "Additional or supplementary" , "Additional, more than expected" .)