Gallop easily (6)
I believe the answer is:
'gallop easily' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for canter that I've seen before include "(Horse's) pace between a trot and a gallop" , "Ride at a quick pace" , "Run" , "Moderate pace for horse" , "Gentle movement in bay, say" .)