One in addition to a horse? (5)
I believe the answer is:
rider
'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'addition to a horse?' is the wordplay.
'addition' becomes 'r' (I can't justify this - if you can you should believe this answer much more).
'to' says to put letters next to each other (I've seen this in other clues).
'a horse?' becomes 'ride'.
'r' after 'ride' is 'RIDER'.
'in' is the link.
Can you help me to learn more?
(Other definitions for rider that I've seen before include "Legal document's additional clause" , "knight" , "Revere, for example" , "Comment from jury" , "Proviso in a legal document" .)