Fastest pace of a horse (6)
I believe the answer is:
gallop
'fastest pace of a horse' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gallop that I've seen before include "'At GPO, all can go at this pace (6)'" , "Fastest run of a horse" , "Fast pace (for horse)" , "Hurry (on horseback?)" , "Fastest pace of horse" .)