Catch small horse (4)

I believe the answer is:

snag

'catch' is the definition.
(snagging is a kind of catching)

'small horse' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'horse' becomes 'nag' (term for an old horse).
's'+'nag'='SNAG'

(Other definitions for snag that I've seen before include "Minor problem" , "Minor hitch" , "Hitch, unexpected problem" , "Disadvantage - small hole in fabric" , "Catch -- problem" .)

I've seen this clue in The Guardian, The Times and The Telegraph.

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