Mournful song when ridge collapsed (5)
I believe the answer is:
dirge
'mournful song' is the definition.
(I've seen this before)
'ridge collapsed' is the wordplay.
'collapsed' is an anagram indicator.
'ridge' is an anagram of 'DIRGE'.
'when' acts as a link.
(Other definitions for dirge that I've seen before include "air?" , "A slow mournful song, lament for the dead" , "Lamentation" , "Dreary song" , "Requiem" .)