It's hot at the end of three months (5)
I believe the answer is:
ember
'end of three months' is the definition.
(September, November and December)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for ember that I've seen before include "Small piece of glowing coal in a dying fire" , "what causes a warm glow" , "Smouldering piece of dying fire" , "Hot piece of wood or coal from a fire" , "Fire remnant" .)