Rocky Mountains lost on large, destructive wave (7)
I believe the answer is:
tsunami
'destructive wave' is the definition.
(I've seen this before)
'rocky mountains lost on' is the wordplay.
'rocky' indicates anagramming the letters (letters rocked or thrown around).
'lost' is a deletion indicator.
'mountains' with 'on' removed is 'mutains'.
'mutains' is an anagram of 'TSUNAMI'.
'large' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for tsunami that I've seen before include "wall of water" , "Large sea wave caused by an earthquake" , "Marine natural disaster" , "High sea wave" , ""Harbour wave"" .)