Mountainside fall (9)
I believe the answer is:
avalanche
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mountainside fall</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)