Derby entrant (5)
(Other definitions for racer that I've seen before include "American snake" , "Greyhound" , "Perhaps 16 [SCRAMBLER]" , "Runner" , "someone driving very fast?" .)
'derby entrant' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for colt that I've seen before include "shooting with it?" , "with a hefty kick?" , "Young player" , "runner" , "pistol" .)