Lament for one buried in cathedral (5)
I believe the answer is:
elegy
'lament' is the definition.
(I know that lament can be written as elegy)
'for one buried in cathedral' is the wordplay.
'for one' becomes 'eg' (both can mean 'for example').
'buried in' means one lot of letters goes inside another.
'cathedral' becomes 'ely' (cathedral city in Cambridgeshire).
'eg' placed into 'ely' is 'ELEGY'.
(Other definitions for elegy that I've seen before include "Plaintive poem" , "Work by Gray?" , "A sorrowful poem like Gray's" , "Requiem" , "Mournful verse" .)