Top of mountain collapses from below causing violent weather (5)
I believe the answer is:
storm
'violent weather' is the definition.
(I've seen this before)
'top of mountain collapses from below' is the wordplay.
'top of' says to take the initial letters.
'collapses' becomes 'rots' (I can't justify this - if you can you should give a lot more credence to this answer).
'from below' says the letters should be written backwards (in a down clue, letters go up).
The initial letter of 'mountain' is 'm'.
'm'+'rots'='mrots'
'mrots' reversed gives 'STORM'.
'causing' acts as a link.
Can you help me to learn more?
(Other definitions for storm that I've seen before include "Trouble" , "commotion" , "Violent outburst" , "Sudden attack" , "Assault (with troops)" .)