Disappointed about others completely trapped in marshy area (11)
I believe the answer is:
crestfallen
'disappointed' is the definition.
(thesaurus)
'others completely trapped in marshy area' is the wordplay.
'others' becomes 'crest' (I can't explain this - if you can you should believe this answer much more).
'completely' becomes 'all' (synonyms).
'trapped in' means one lot of letters goes inside another.
'marshy area' becomes 'fen' (both can mean wetland).
'all' inserted into 'fen' is 'fallen'.
'crest'+'fallen'='CRESTFALLEN'
'about' acts as a link.
Can you help me to learn more?
(Other definitions for crestfallen that I've seen before include "Sad, disappointed" , "Downcast" , "Sadly disappointed" , "Feeling a blow to one's pride" , "Deflated" .)