Horse's insect problem making a comeback (7)
I believe the answer is:
mustang
'horse's' is the definition.
(mustang is a kind of horse)
'insect problem making a comeback' is the wordplay.
'insect' becomes 'gnat' (gnat is a kind of insect).
'problem' becomes 'sum' (arithmetic problem).
'making a comeback' is a reversal indicator.
'gnat'+'sum'='gnatsum'
'gnatsum' reversed gives 'MUSTANG'.
(Other definitions for mustang that I've seen before include "Small, half-wild horse of the U.S." , "creature" , "one running wild" , "Small, wild US horse" , "Small feral horse of the US" .)