Volume is difficult to support (8)
I believe the answer is:
hardback
'volume' is the definition.
(I know that hardback is a type of volume)
'difficult to support' is the wordplay.
'difficult' becomes 'hard' (synonyms).
'to support' becomes 'back' (to back is to support or endorse).
'hard'+'back'='HARDBACK'
'is' is the link.
(Other definitions for hardback that I've seen before include "Penguins are rarely so" , "Style of book binding" , "Sort of book" , "It might be a novel!" , "Book with board covers" .)