Significant stuff (8)
I believe the answer is:
material
I believe this clue is a double definition.
'significant' is the first definition.
(thesaurus)
'stuff' is the second definition.
('material' can be a synonym of 'stuff')
(Other definitions for material that I've seen before include "Matter; fabric" , "of serious importance" , "Relevant - fabric" , "Fabric or other substance" , "Substantial -- textile" .)