Race often won by such effrontery (4)
I believe the answer is:
neck
I believe this is a double definition.
'race often won by such' is the first definition.
(to win by a neck is to win by a narrow margin)
'effrontery' is the second definition.
(neck can mean effrontery or impudence)
(Other definitions for neck that I've seen before include "Body part and the nerve to do something" , "It's risky to stick it out" , "top of bottle" , "Bottle part" , "jowl?" .)