Important horse race (7)
I believe the answer is:
classic
'important horse race' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for classic that I've seen before include "Important horse race" , "Flat race" , "Of the highest order - the Derby, for example" , "Oaks perhaps" , "Epsom Derby or St Leger Stakes, for example" .)