To the horse, something added (5)
I believe the answer is:
rider
'to the' is the definition.
I can't judge whether this defines the answer.
'horse something added' is the wordplay.
'horse' becomes 'ride' (animal that can be ridden).
'something' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'added' is a charade indicator (letters next to each other).
'ride'+'r'='RIDER'
Can you help me to learn more?
(Other definitions for rider that I've seen before include "Equestrian added by a jury?" , "Horseman or supplementary jury verdict" , "One backing a horse" , "Horseman or add-on verdict by jury" , "Supplement to verdict" .)