American coach chasing very big award (5)
I believe the answer is:
oscar
'award' is the definition.
(Academy Award)
'american coach chasing very big' is the wordplay.
'american coach' becomes 'car' (US term for a train coach).
'chasing' means one lot of letters go next to another.
'very big' becomes 'os' (abbreviation for outsize).
'car' put after 'os' is 'OSCAR'.
(Other definitions for oscar that I've seen before include "Note" , "letter read out on radio" , "Film industry award" , "Wildean film award?" , "Annual film award" .)