Happen in mountain route (4)
I believe the answer is:
pass
This clue is a double definition.
'happen' is the first definition.
(I know that happen is a more specific form of the action happen)
'mountain route' is the second definition.
(a mountain pass)
'in' acts as a link.
(Other definitions for pass that I've seen before include "I've no idea" , "complimentary ticket" , "Exceed; leave behind" , "Exam success" , "Mountain road" .)