Fast gait (6)
I believe the answer is:
gallop
'fast gait' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gallop that I've seen before include "'Run at a high speed, like a horse (6)'" , "All at GPO can go at this rate" , "Fastest pace of horse" , "go quickly" , "Go at great speed" .)