Small midge problem upset horses (8)

I believe the answer is:

mustangs

'horses' is the definition.
(mustang is a kind of horse)

'small midge problem upset' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'midge' becomes 'gnat' (I've seen this before).
'problem' becomes 'sum' (arithmetic problem).
'upset' shows that the letters should be reversed in order.
's'+'gnat'+'sum'='sgnatsum'
'sgnatsum' reversed gives 'MUSTANGS'.

(Another definition for mustangs that I've seen is " Small half-wild horses".)

I've seen this clue in The Telegraph.

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