Then not a hot number? (3)
I believe the answer is:
ten
'number?' is the definition.
(ten is a number)
'then not a hot' is the wordplay.
'not' is a deletion indicator.
'a hot' becomes 'h' (abbreviation eg on taps).
'then' with 'h' removed is 'TEN'.
(Other definitions for ten that I've seen before include "Five times two" , "Base of decimal system" , "No of sides of a decagon" , "X-WORD" , "Cardinal number" .)