Race in the amphitheatre (4)
'race' is the definition.
(I've seen this before)
'in the amphitheatre' is the wordplay.
'in' says the answer is hidden in the clue.
'HEAT' can be found hidden inside 'amphitheatre'.
(Other definitions for heat that I've seen before include "Warm up", "Energy that caues a rise in temperature", "Pressure", "Warmth - gossip mag", "It's measured in therms".)