Material thats great, opulent mostly (6)
I believe the answer is:
fabric
'material' is the definition.
('fabric' can be a synonym of 'material')
'great opulent mostly' is the wordplay.
'great' becomes 'fab' (I've seen this in other clues).
'opulent' becomes 'rich' ('rich' can be similar in meaning to 'opulent').
'mostly' means to remove the last letter.
'rich' with its last letter taken off is 'ric'.
'fab'+'ric'='FABRIC'
'thats' is the link.
(Other definitions for fabric that I've seen before include "Building structure; textile" , "Woven or knitted material" , "Knitted or woven fabric" , "Cloth, woven material" , "Perhaps felt" .)