A good part of three months and it's still warm (5)

I believe the answer is:

ember

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'a good part of three months' is the definition.
(September, November, December)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ember that I've seen before include "Fire remnant" , "Smouldering piece left over from a fire" , "Hot piece of wood or coal from a fire" , "Glowing coal; prayer (Days)" , "Cryptic question" .)

I've seen this clue in The Guardian.

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