Special fireplace is unique (6)
I believe the answer is:
single
'unique' is the definition.
(I've seen this before)
'special fireplace' is the wordplay.
'special' becomes 's' (common abbreviation as in Special Air Service).
'fireplace' becomes 'ingle' (I've seen this before).
's'+'ingle'='SINGLE'
'is' acts as a link.
(Other definitions for single that I've seen before include "Unaccompanied" , "Unattached" , "one > individual" , "Only one; record" , "Just one - on one's own" .)