A clan have dreadful downfall in the Cairngorms, perhaps (9)
I believe the answer is:
avalanche
'downfall in the cairngorms perhaps' is the definition.
(the Cairngorms are a mountain range)
'a clan have dreadful' is the wordplay.
'dreadful' is an anagram indicator.
'clan'+'have'='clanhave'
'clanhave' anagrammed gives 'valanche'.
'a'+'valanche'='AVALANCHE'
(Other definitions for avalanche that I've seen before include "Snow slip" , "Rapid movement of a large mass of snow or ice" , "Great snowfall" , "Come down heavily" , "Big slide of snow or stone" .)