One small fireplace (6)
I believe the answer is:
single
'one' is the definition.
(synonyms)
'small fireplace' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fireplace' becomes 'ingle' (I've seen this before).
's'+'ingle'='SINGLE'
(Other definitions for single that I've seen before include "Unaccompanied" , "one > individual" , "Free" , "Still miss" , "Short pop record" .)