Eccentric bishop with a lot of sumptuous material (7)
I believe the answer is:
cambric
'material' is the definition.
(cambric is a kind of material)
'eccentric bishop with a lot of sumptuous' is the wordplay.
'eccentric' becomes 'cam' (an eccentric is a mechanical device such as a cam).
'bishop' becomes 'b' (abbreviation used in chess).
'with' means one lot of letters go next to another.
'a lot of' 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 last letter removed is 'ric'.
'cam'+'b'+'ric'='CAMBRIC'
(Other definitions for cambric that I've seen before include "Material" , "Lightweight linen or cotton fabric" , "Lightweight cotton or linen cloth" , "Fine white fabric" , "Fine white linen" .)