Check on the horse (4)
I believe the answer is:
rein
'check' is the definition.
(both can mean to control or limit)
'on the horse' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'the horse' becomes 'in' (I can't justify this - if you can you should believe this answer much more).
're'+'in'='REIN'
Can you help me to learn more?
(Other definitions for rein that I've seen before include "Check; control" , "Part of a horses's harness" , "Director's power" , "Free ... is carte blanche" , "Hold back" .)