Stop with the firm (7)
I believe the answer is:
staunch
I believe this clue is a double definition.
'stop' is the first definition.
(staunching is a kind of stopping)
'the firm' is the second definition.
(thesaurus)
'with' acts as a link.
(Other definitions for staunch that I've seen before include "Stay - faithful" , "Dependable; stop (flow)" , "Sturdy" , "Reliable" , "Very loyal; stop (a flow of blood)" .)