Some of the athletes in preliminary race (4)
I believe the answer is:
heat
'preliminary race' is the definition.
(I've seen this before)
'some of the athletes in' is the wordplay.
'some of' says the answer is hidden in the clue.
'HEAT' can be found hidden inside 'the athletes in'.
(Other definitions for heat that I've seen before include "eliminating round" , "Preliminary race to qualify finalists" , "Warmth - gossip mag" , "Raise temperature" , "High temperature" .)