Track trials (5)
I believe the answer is:
heats
'track trials' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for heats that I've seen before include "Makes hot or warm" , "Haste (anag.)" , "Eliminating rounds in a contest" , "Warms up for the preliminary races" , "Eliminating rounds of a race" .)