Horse’s fastest pace (6)
I believe the answer is:
gallop
'horse's fastest pace' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gallop that I've seen before include "Horse's fastest pace" , "A fast canter" , "'Run at a high speed, like a horse (6)'" , "Fast gait of a horse" , "Fastest run of a horse" .)