Qualifying races (5)
I believe the answer is:
heats
'qualifying 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 "Warms up for the preliminary races" , "Eliminating rounds in a contest" , "Eliminating rounds of a race" , "Makes hot or warm" , "Haste (anag.)" .)