Happen to fall (5)
I believe the answer is:
occur
This clue is a double definition.
'happen' is the first definition.
(synonyms)
'fall' is the second definition.
(thesaurus)
'to' is the link.
(Other definitions for occur that I've seen before include "found" , "exist" , "crop up" , "Spring to mind" , "Happen, take place" .)