All-important fault-finding (8)
I believe the answer is:
critical
I believe this is a double definition.
'all-important' is the first definition.
(I've seen this before)
'fault-finding' is the second definition.
(I've seen this before)
(Other definitions for critical that I've seen before include "Finding faults" , "Decisively important" , "essential" , "Urgent - fault-finding" , "Urgent or carping" .)