Sofa is hard to support (6)
I believe the answer is:
settee
'sofa' is the definition.
(settee is a kind of sofa)
'hard to support' is the wordplay.
'hard' becomes 'set' ('set' can be similar in meaning to 'hard').
'to support' becomes 'tee' (I've seen this before).
'set'+'tee'='SETTEE'
'is' acts as a link.
(Other definitions for settee that I've seen before include "Item of furniture" , "Sofa, couch" , "Multiple seat" , "A long couch without arm-rests" , "Long upholstered seat with a back" .)