Poem, say, inscribed in cathedral (5)
I believe the answer is:
Here is my best explanation:
'poem' is the definition.
(elegy is a kind of poem)
'say inscribed in cathedral' is the wordplay.
'say' becomes 'eg'.
'inscribed in' is an insertion indicator.
'cathedral' becomes 'ely' (ely is a cathedral city in cambridgeshire).
'eg' put into 'ely' is 'ELEGY'.
(Other definitions for elegy that I've seen before include "Sorrowful poem/song", "Requiem", "Sad or reflective poem", "that may be on the dead", "Sad work", "Poem penned beside church", "Gray work", "Mournful work", "Poem lamenting the dead" and "Milton's Lycidas?".)