Run down lair with one fireplace (9)
I believe the answer is:
denigrate
'run down' is the definition.
(I've seen this in another clue)
'lair with one fireplace' is the wordplay.
'lair' becomes 'den' (synonyms).
'with' is a charade indicator (letters next to each other).
'one' becomes 'I' (Roman numeral).
'fireplace' becomes 'grate' (grate can mean a fireplace).
'den'+'i'+'grate'='DENIGRATE'
(Other definitions for denigrate that I've seen before include "Bad-mouth" , "Blacken" , "Disparage" , "Unfairly criticise" , "Defame" .)