"500” auto race (4)
I believe the answer is:
indy
'500 auto race' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for indy that I've seen before include "US motor-racing" , "Racing in fast cars" , "Abandon, discard" , "US form of motor racing" , "500-mile US motorcar race" .)