Track races (5)
'track races' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for heats that I've seen before include "Eliminating rounds of a race" , "Eliminating rounds in a contest" , "Haste (anag.)" , "Warms up for the preliminary races" , "Makes hot or warm" .)
(Other definitions for relays that I've seen before include "Foot races in which batons are passed" , "Passes on, like baton in race" , "Transmits" , "Broadcasts" , "Passes on in multi-runner races" .)