Serious fault-finding (8)
I believe the answer is:
critical
I believe this clue is a double definition.
'serious' is the first definition.
(I've seen this before)
'fault-finding' is the second definition.
(thesaurus)
(Other definitions for critical that I've seen before include "Crucial, decisive" , "like one in an armchair perhaps" , "At crucial stage, or being disparaging" , "Decisive - fault-finding" , "Urgent or carping" .)