Brilliant, mostly sumptuous material (6)
I believe the answer is:
fabric
'material' is the definition.
(I know that material can be written as fabric)
'brilliant mostly sumptuous' is the wordplay.
'brilliant' becomes 'fab' (short for fabulous).
'mostly' means to remove the last letter (most but not all of the word).
'sumptuous' becomes 'rich' (similar in meaning).
'rich' with its last letter taken away is 'ric'.
'fab'+'ric'='FABRIC'
(Other definitions for fabric that I've seen before include "Knitted or woven fabric" , "Cotton or tweed, say" , "(Texture of) cloth" , "Woven or knitted material" , "Material in general" .)