Happens to arrive somewhere in the mountains carrying small son (5,2,4)
I believe the answer is:
comes to pass
'happens' is the definition.
('come to pass' can be a synonym of 'happen')
'to arrive somewhere in the mountains carrying small son' is the wordplay.
'arrive' becomes 'come' (synonyms).
'somewhere in the mountains' becomes 'spas' (I can't justify this - if you can you should give a lot more credence to this answer).
'carrying' is an insertion indicator.
'small son' becomes 's' (genealogical abbreviation for son; 'small' indicates a short form).
'come'+'spas'='comespas'
'to' placed inside 'comespas' is 'comestopas'.
'comestopas'+'s'='COMES TO PASS'
Can you help me to learn more?
(Another definition for comes to pass that I've seen is " Happens".)