Crossword Genius

Long ago got onto small problem for horse (4,4)

Ross

I believe the answer is:

sway-back

'problem for horse' is the definition.
(back problem found in some horses)

'long ago got onto small' is the wordplay.
'long ago' becomes 'wayback'.
'got onto' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'wayback' after 's' is 'SWAY-BACK'.

I've seen this clue in The Times.
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