Rider's request upset Scotsman (10)
I believe the answer is:
equestrian
'rider's' is the definition.
(equestrian is a kind of rider)
'request upset scotsman' is the wordplay.
'upset' indicates anagramming the letters.
'scotsman' becomes 'ian' (common Scottish name).
'request' with letters rearranged gives 'equestr'.
'equestr'+'ian'='EQUESTRIAN'
(Other definitions for equestrian that I've seen before include "Horse-rider" , "Jockey" , "Horse rider" , "Person on horseback" , "Horseman" .)