Checks on the horse (5)
I believe the answer is:
reins
'checks' is the definition.
(reining is a kind of checking)
'on the horse' is the wordplay.
'on' becomes 're' (regarding).
'the horse' becomes 'ins' (I can't justify this - if you can you should believe this answer much more).
're'+'ins'='REINS'
Can you help me to learn more?
(Other definitions for reins that I've seen before include "Means of control for a horse" , "Straps for controlling a horse" , "Power to direct and control" , "water regulators?" , "Control for horses" .)