Not all of the athletes in preliminary race (4)

I believe the answer is:

heat

'preliminary race' is the definition.
(I've seen this before)

'not all of the athletes in' is the wordplay.
'not all of' indicates a hidden word (I've seen 'not all' mean this).
'HEAT' is hidden within 'the athletes in'.

(Other definitions for heat that I've seen before include "Hotness" , "Level of spice" , "Preliminary race to qualify finalists" , "eliminating round" , "Raise temperature" .)

I've seen this clue in The Times.

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