Where coals are not hot, mix with others (9)
I believe the answer is:
integrate
'hot mix with others' is the definition.
I can't tell whether this defines the answer.
'where coals are not' is the wordplay.
I cannot really see how this works, but
'are' could be 'a' (the 'are' is a metric unit of area) and 'a' is present in the answer.
'not' could be 'nt' ('n't' can be a synonym of 'not') and 'nt' is found in the answer.
This explanation may well be incorrect...
(Other definitions for integrate that I've seen before include "Get together" , "unify" , "Organise a get-together" , "Incorporate into the main group" , "Incorporate into the whole" .)