Rat's fault (6)
I believe the answer is:
defect
I believe this is a double definition.
'rat's' is the first definition.
(thesaurus)
'fault' is the second definition.
(I've seen this before)
(Other definitions for defect that I've seen before include "Want or imperfection" , "Join opponents in a cause" , "Abandon one's country in favour of an opposing one" , "'Flaw, imperfection (6)'" , "change allegiance" .)