Reg and I'd composed funeral hymn (5)
I believe the answer is:
'funeral hymn' is the definition.
(I've seen this in another clue)
'reg and i'd composed' is the wordplay.
'and' is a charade indicator (letters next to each other).
'composed' indicates anagramming the letters.
'reg' after 'id' is 'idreg'.
'idreg' anagrammed gives 'DIRGE'.
(Other definitions for dirge that I've seen before include "Lamentation" , "Dragging piece" , "lamentable composition" , "dreary tune" , "Funeral song" .)