Finding fault is crucial (8)
I believe the answer is:
critical
I believe this is a double definition.
'finding fault' is the first definition.
(I've seen this before)
'crucial' is the second definition.
(similar in meaning)
'is' acts as a link.
(Other definitions for critical that I've seen before include "Adversely commenting" , "of such crucial importance" , "Key" , "Urgent or carping" , "Decisive - fault-finding" .)