Break down or fail (7)
I believe the answer is:
founder
I believe this clue is a double definition.
'break down' is the first definition.
(thesaurus)
'fail' is the second definition.
(foundering is a kind of failing)
(Other definitions for founder that I've seen before include "Beginner" , "(Of ship) fill with water and sink" , "One who makes an institution" , "Sink like originator?" , "One sets up firm" .)