Reduced prize for entering afternoon heat in advance (7)

'heat in advance' is the definition.

'reduced prize for entering afternoon' is the wordplay.
'reduced' means to remove the last letter.
'prize' becomes 'reward' (I've seen this before).
'for entering' is an insertion indicator.
'afternoon' becomes 'pm'.
'reward' with its last letter removed is 'rewar'.
'rewar' going into 'pm' is 'PREWARM'.