Fly problem upset horse (7)
I believe the answer is:
mustang
'horse' is the definition.
(mustang is a kind of horse)
'fly problem upset' is the wordplay.
'fly' becomes 'gnat' (I've seen this before**).
'problem' becomes 'sum' (arithmetic problem).
'upset' shows that the letters should be reversed in order.
'gnat'+'sum'='gnatsum'
'gnatsum' written backwards gives 'MUSTANG'.
(Other definitions for mustang that I've seen before include "Wild or half-wild horse" , "creature" , "Trotter" , "Feral American horse" , "Small, half-wild horse of the U.S." .)