Massive grave (7)
I believe the answer is:
weighty
I believe this is a double definition.
'massive' is the first definition.
('weighty' can be similar in meaning to 'massive')
'grave' is the second definition.
(thesaurus)
(Other definitions for weighty that I've seen before include "Important" , "substantial" , "having great influence" , "Heavy - very serious" , "Cumbersome" .)