Happen to disagree (4,3)
I believe the answer is:
fall out
This clue is a double definition.
'happen' is the first definition.
(thesaurus)
'disagree' is the second definition.
(thesaurus)
'to' acts as a link.
(Other definitions for fall out that I've seen before include "Quarrel - leave the ranks" , "Happen" , "disagree" , "Radioactive pollution" .)