Races one can't complete (6)
I believe the answer is:
relays
'races one can't complete' is the definition.
(team race so cannot be done by an individual)
This is all the clue.
(Other definitions for relays that I've seen before include "Passes on a message" , "Transmits" , "Receives and passes on" , "Races for teams of four" , "teams of relief workers" .)