Switch to a horse for half a mile! (6)
I believe the answer is:
toggle
'switch' is the definition.
(I know that toggle is a type of switch)
'to a horse for half a mile' is the wordplay.
'a horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'for' is a charade indicator (letters next to each other).
'half a' indicates taking half.
'mile' cut in half is 'le'.
'to'+'gg'+'le'='TOGGLE'
(Other definitions for toggle that I've seen before include "Switch (between states)" , "Hinged switch" , "Fastener; type of switch" , "Wooden pin" , "On-off switch" .)