Racehorse's gait (6)
I believe the answer is:
gallop
'racehorse's gait' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gallop that I've seen before include "Fast ride" , "All at GPO can go at this rate" , "Horse's gait" , "Horse's fastest pace" , "Fast speed of horse" .)