Cloth is wonderful: sumptuous, almost (6)
I believe the answer is:
fabric
'cloth' is the definition.
('fabric' can be a synonym of 'cloth')
'wonderful sumptuous almost' is the wordplay.
'wonderful' becomes 'fab' (slang abbreviation of fabulous).
'sumptuous' becomes 'rich' ('rich' can be similar in meaning to 'sumptuous').
'almost' means to remove the last letter (most of the word but not all of it).
'rich' with its final letter removed is 'ric'.
'fab'+'ric'='FABRIC'
'is' acts as a link.
(Other definitions for fabric that I've seen before include "Building structure; textile" , "Cloth, woven material" , "(Texture of) cloth" , "Woven or knitted material" , "Perhaps felt" .)