Dejected as ridge collapsed (11)
I believe the answer is:
crestfallen
'dejected' is the definition.
('crestfallen' can be similar in meaning to 'dejected')
'ridge collapsed' is the wordplay.
'ridge' becomes 'crest' (I've seen this before).
'collapsed' becomes 'fallen'.
'crest'+'fallen'='CRESTFALLEN'
'as' acts as a link.
(Other definitions for crestfallen that I've seen before include "Dejected after blow to pride" , "Downcast" , "Deflated" , "Sadly disappointed" , "Sad, disappointed" .)