One's attached to horse (5)

I believe the answer is:

pinto

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'horse' is the definition.
(pinto is a kind of horse)

'one's attached to' is the wordplay.
'one's' becomes 'pin' (I can't justify this - if you can you should believe this answer much more).
'attached' means one lot of letters go next to another.
'pin'+'to'='PINTO'

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(Other definitions for pinto that I've seen before include "Piebald horse (US)" , "H [HORSE]" , "A piebald pony" , "Spotted or piebald horse" , "patchy horse" .)

I've seen this clue in The Independent.

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