Warms up for rounds of races (5)
I believe the answer is:
heats
This clue could be a double definition.
'warms up' is the first definition.
(I've seen this before)
'rounds of races' is the second definition.
'heats' can be an answer for 'races' (I've seen this before). I am not certain of the 'rounds of' bit.
'for' is the link.
Can you help me to learn more?
(Other definitions for heats that I've seen before include "Makes hot or warm" , "Eliminating rounds in a contest" , "Warms up for the preliminary races" , "Haste (anag.)" , "Eliminating rounds of a race" .)