Rider in hunt cutting a rein to pieces (10)
I believe the answer is:
equestrian
'rider' is the definition.
(I know that equestrian is a type of rider)
'hunt cutting a rein to pieces' is the wordplay.
'hunt' becomes 'quest' (quest is a kind of hunt).
'cutting' means one lot of letters goes inside another (some letters cut their way into a word).
'to pieces' indicates anagramming the letters (the same pieces in a new order).
'a'+'rein'='arein'
'arein' with letters rearranged gives 'erian'.
'quest' inserted within 'erian' is 'EQUESTRIAN'.
'in' acts as a link.
(Other definitions for equestrian that I've seen before include "Jockey" , "Relating to horse riding" , "Horseman" , "Horse rider" , "Person on horseback" .)