Failing to turn traitor (6)
I believe the answer is:
defect
I believe this clue is a double definition.
'failing' is the first definition.
(thesaurus)
'turn traitor' is the second definition.
'to' acts as a link.
(Other definitions for defect that I've seen before include "Flaw; turn traitor" , "change allegiance" , "Leave the country" , "error" , "Desert and join the other side" .)