An add-on in the next race (5)
I believe the answer is:
extra
'an add-on' is the definition.
(I've seen this in another clue)
'in the next race' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EXTRA' is hidden within 'next race'.
(Other definitions for extra that I've seen before include "lesser player" , "Crowd-scene actor" , "to boot" , "Not a standard item" , "like one of the crowd" .)