Derby racers (6)
I believe the answer is:
horses
'derby racers' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for horses that I've seen before include "Equine animals" , "Equine beasts" , "Eg, stallions" , "Hoofed mammals (which go with 19 4)" , "''If wishes were . . . . . ., beggars would ride''" .)