Happen to arrive at route through mountains (4,2,4)
I believe the answer is:
come to pass
'happen' is the definition.
('come to pass' can be a synonym of 'happen')
'arrive at route through mountains' is the wordplay.
'arrive at' becomes 'cometo' (I've seen this before).
'route through mountains' becomes 'pass' (I've seen this before).
'cometo'+'pass'='COME TO PASS'
'to' acts as a link.
(Other definitions for come to pass that I've seen before include "Short military stick" , "Take place" , "reach mountain road" , "Biblically happen" .)