Weighty stuff (8)
I believe the answer is:
material
I believe this clue is a double definition.
'weighty' is the first definition.
(thesaurus)
'stuff' is the second definition.
(I know that material is a type of stuff)
(Other definitions for material that I've seen before include "Fabric, stuff" , "Preoccupied with physical, rather than spiritual, substance" , "grave" , "I alert ma (anag.)" , "Essential" .)