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