Horseman mounted donkey in turn to one side (7)
I believe the answer is:
cossack
'horseman' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cossack that I've seen before include "One of an elite cavalry corps in tzarist Russia" , "Tsarist cavalryman" , "Member of Russian, Ukrainian people" , "Russian tribesman famed for horsemanship" , "One from southern Russia or Ukraine noted for horse riding skills" .)