Horse fly, then result is rearing (7)
I believe the answer is:
mustang
'horse' is the definition.
(I know that mustang is a horse breed)
'fly then result is rearing' is the wordplay.
'fly' becomes 'gnat' (type of insect).
'then result' becomes 'sum' (I've seen this in another clue. I am not sure about the 'then' bit.).
'is rearing' is a reversal indicator (as in a horse rearing up).
'gnat'+'sum'='gnatsum'
'gnatsum' back-to-front is 'MUSTANG'.
Can you help me to learn more?
(Other definitions for mustang that I've seen before include "Wild or half-wild horse" , "one running wild" , "Feral American horse" , "Small, half-wild horse of the U.S." , "SW US wild horse" .)