In a gallop, ace rode a horse (5)
I believe the answer is:
pacer
'horse' is the definition.
(a horse which paces)
'in a gallop ace rode a' is the wordplay.
'in' says the answer is hidden in the clue.
'PACER' is hidden in the letters of 'gallop ace rode a'.
(Other definitions for pacer that I've seen before include "Competitor" , "Horse; one setting speed" , "Racing horse with an unusual gait" , "walker?" , "early race leader?" .)