Ride fast, going away at first — then head back (6)
I believe the answer is:
'ride fast' is the definition.
(I've seen this before)
'going away at first then head back' is the wordplay.
'at first' suggests taking the first letters.
'then' is a charade indicator (letters next to each other).
'head' becomes 'poll' (poll is a now rare word for head or scalp**).
'back' shows that the letters should be reversed in order.
The initial letters of 'going away' is 'ga'.
'poll' back-to-front is 'llop'.
(Other definitions for gallop that I've seen before include "Move fast" , "Fast speed of horse" , "Fast gait of a horse" , "Faster than a canter" , "Fastest run of a horse" .)