Stop being faithful (7)
I believe the answer is:
staunch
I believe this is a double definition.
'stop' is the first definition.
(staunching is a kind of stopping)
'faithful' is the second definition.
(thesaurus)
'being' is the link.
(Other definitions for staunch that I've seen before include "act to stop bleeding" , "Plug" , "solid" , "Check" , "Hun acts in firm and loyal way" .)