Crossword Genius

Fly problem upset horse (7)

Ross

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." .)

I've seen this clue in The Telegraph.
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