He doesn’t believe the heat is off, at first (7)
I believe the answer is:
atheist
'he doesn't believe' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'heat is off at first' is the wordplay.
'is off' indicates anagramming the letters.
'at' is a charade indicator (letters next to each other).
'first' becomes 'ist' (ist resembles 1st).
'heat' anagrammed gives 'athe'.
'athe'+'ist'='ATHEIST'
'the' is the link.
Can you help me to learn more?
(Other definitions for atheist that I've seen before include "One denies the existence of gods" , "Unbeliever" , "One denies the existence of a god" , "Unfaithful type" , "One who believes there is no god" .)