For one buried in fenland see mournful poem (5)
I believe the answer is:
elegy
'mournful poem' is the definition.
(I've seen this in another clue)
'for one buried in fenland see' is the wordplay.
I cannot really see how this works, but
'for one' could be 'eg' (both can mean 'for example') and 'eg' is found in the answer.
'see' could be 'ely' (see is a place with a cathedral - e.g. Ely) and 'ely' is found in the leftover letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for elegy that I've seen before include "Sad or reflective poem" , "Poem lamenting the dead" , "Sad work" , "Poetic lament for the dead" , "Dirge" .)