Material's marvellous, almost sumptuous (6)
I believe the answer is:
fabric
'material's' is the definition.
('fabric' can be a synonym of 'material')
'marvellous almost sumptuous' is the wordplay.
'marvellous' becomes 'fab' (slang abbreviation of fabulous).
'almost' means to remove the last letter (most of the word but not all of it).
'sumptuous' becomes 'rich' ('rich' can be similar in meaning to 'sumptuous').
'rich' with its final letter taken off is 'ric'.
'fab'+'ric'='FABRIC'
(Other definitions for fabric that I've seen before include "Woven or knitted material" , "Woven textile" , "Silk or satin, say" , "stuff" , "Building structure; textile" .)