Reminder for one horse to be mounted (5)
I believe the answer is:
nudge
'reminder' is the definition.
(I've seen this before)
'for one horse to be mounted' is the wordplay.
'for one' becomes 'eg' (both can mean 'for example').
'horse' becomes 'dun' (I've seen this before).
'to be mounted' says the letters should be written backwards (in a down clue, letters mount up).
'eg'+'dun'='egdun'
'egdun' written backwards gives 'NUDGE'.
(Other definitions for nudge that I've seen before include "A gentle reminder?" , "Jogger" , "Gentle encouragement" , "Light touch or push" , "Jog" .)