Jog back for one horse (5)
I believe the answer is:
nudge
'jog' is the definition.
(I know that jog can be written as nudge)
'back for one horse' is the wordplay.
'back' shows that the letters should be reversed in order.
'for one' becomes 'eg' (both can mean 'for example').
'horse' becomes 'dun' (I've seen this before).
'eg'+'dun'='egdun'
'egdun' back-to-front is 'NUDGE'.
(Other definitions for nudge that I've seen before include "Shove" , "Gentle push" , "Encouraging prompt" , "Gently encourage" , "Push (with a wink?)" .)