Sudden fall of snow disrupted a mostly calm haven (9)
I believe the answer is:
avalanche
'sudden fall of snow' is the definition.
I know that 'snow' relates to this answer.
'disrupted a mostly calm haven' is the wordplay.
'disrupted' is an anagram indicator.
'mostly' means to remove the last letter (most but not all of the word).
'calm' with its final letter taken off is 'cal'.
'a'+'cal'+'haven'='acalhaven'
'acalhaven' with letters rearranged gives 'AVALANCHE'.
Can you help me to learn more?
(Other definitions for avalanche that I've seen before include "Great snowfall" , "Rapid movement of a large mass of snow or ice" , "Come down heavily" , "Danger" , "Large mass of sliding snow or stone" .)