Horseman may give an ace turn (7)
I believe the answer is:
centaur
'horseman' is the definition.
(half horse, half man)
'may give an ace turn' is the wordplay.
'may' indicates an anagram (I've seen 'might' mean this (I've seen this in other clues)).
'give' says to put letters next to each other.
'ace' next to 'turn' is 'aceturn'.
'aceturn' is an anagram of 'CENTAUR'.
(Other definitions for centaur that I've seen before include "Horsey person" , "Mythical man-horse" , "Fabulous beast, half man and half horse" , "Mythological horse-man" , "'Mythical creature, half man, half horse (7)'" .)