Finding fault very important (8)
I believe the answer is:
critical
This clue is a double definition.
'finding fault' is the first definition.
(I've seen this before)
'very important' is the second definition.
(I've seen this before)
(Other definitions for critical that I've seen before include "Finding faults" , "expressing disapproving comments" , "At crucial stage, or being disparaging" , "Decisive - fault-finding" , "serious" .)