Gallop from Aintree racehorse when mounted (6)
I believe the answer is:
career
'gallop' is the definition.
(I've seen this before)
'from aintree racehorse when mounted' is the wordplay.
'from' indicates a hidden word.
'mounted' says the letters should be written backwards.
'reerac' is hidden within 'aintree racehorse when'.
'reerac' written backwards gives 'CAREER'.
(Other definitions for career that I've seen before include "History of working life (6)" , "Run - profession" , "Progress through work" , "Occupation through time" , "making progress" .)