Mingle for winter inside part of fireplace (9)
I believe the answer is:
integrate
'mingle' is the definition.
(I know that combine is the is a more specific form of the action of integrate.)
'winter inside part of fireplace' is the wordplay.
'inside' means to look at the middle letters.
'part of fireplace' becomes 'grate' (I've seen this before).
The centre of 'winter' is 'inte'.
'inte'+'grate'='INTEGRATE'
'for' acts as a link.
(Other definitions for integrate that I've seen before include "unify" , "Incorporate into the main group" , "Combine into a whole, unite" , "Incorporate into the whole" , "Mix" .)