Strengthened control for horse used unnaturally (10)

I believe the answer is:

reinforced

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'strengthened' is the definition.
(synonyms)

'control for horse used unnaturally' is the wordplay.
'control for horse' becomes 'rein' (I've seen this before).
'used unnaturally' becomes 'forced' (I can't explain this - if you can you should believe this answer much more).
'rein'+'forced'='REINFORCED'

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(Other definitions for reinforced that I've seen before include "Underlined" , "Added to" , "Sort of concrete" , "Strengthened" .)

I've seen this clue in The Irish News.

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