Mountainside fall (9)
I believe the answer is:
avalanche
'mountainside fall' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for avalanche that I've seen before include "Slide of large amounts of snow or stone" , "Rapid movement of a large mass of snow or ice" , "Rock-fall" , "Snowslip" , "Large fall of snow or rocks" .)