Item of furniture's firm support (6)
I believe the answer is:
settee
'item of furniture's' is the definition.
(I've seen this before)
'firm support' is the wordplay.
'firm' becomes 'set' (I've seen this before).
'support' becomes 'tee' (I've seen this before).
'set'+'tee'='SETTEE'
(Other definitions for settee that I've seen before include "two-seater?" , "Intended recipient of my work, so to speak" , "Seat for more than one" , "A long couch without arm-rests" , "Item of furniture" .)