Blacken as one occupying room with fireplace (9)
I believe the answer is:
denigrate
'blacken' is the definition.
(thesaurus)
'one occupying room with fireplace' is the wordplay.
'one' becomes 'i' (Roman numeral).
'occupying' means one lot of letters goes inside another.
'room' becomes 'den' (den is a kind of room**).
'with' says to put letters next to each other.
'fireplace' becomes 'grate' (I've seen this before).
'den'+'grate'='dengrate'
'i' put within 'dengrate' is 'DENIGRATE'.
'as' is the link.
(Other definitions for denigrate that I've seen before include "Unfairly criticise" , "Slight" , "put down" , "Disparage" , "Criticise, belittle" .)