Major finding fault with grave (8)
I believe the answer is:
critical
This clue is a triple definition!
'major' is the first definition.
(I've seen this before)
'finding fault' is the second definition.
(I've seen this before)
'with' acts as a link.
'grave' is the third definition.
(similar in meaning)
(Other definitions for critical that I've seen before include "Disapproving; extremely ill" , "Crucial, decisive" , "Decisively important" , "At crucial stage, or being disparaging" , "Expressing disapproval" .)