Fall on mountain — see man in pain (9)
I believe the answer is:
avalanche
'fall on mountain' is the definition.
'avalanche' can be an answer for 'fall' (I've seen this before). I'm unsure of the 'on mountain' bit.
'see man in pain' is the wordplay.
'see' becomes 'v' (short for 'vide', Latin meaning 'see elsewhere').
'man' becomes 'alan' ().
'in' indicates putting letters inside.
'pain' becomes 'ache' (ache is a kind of pain).
'v'+'alan'='valan'
'valan' placed within 'ache' is 'AVALANCHE'.
Can you help me to learn more?
(Other definitions for avalanche that I've seen before include "Great snowfall" , "heavy snowfall" , "Sudden descent of massed snow etc" , "Slide of large amounts of snow or stone" , "Disastrous snowfall" .)