Funeral song to stop incorporating reflection of old King (5)
I believe the answer is:
dirge
'funeral song' is the definition.
(I've seen this before)
'stop incorporating reflection of old king' is the wordplay.
'stop' becomes 'die' (as in a machine stopping or dying).
'incorporating' means one lot of letters goes inside another.
'reflection of' says the letters should be written backwards (I've seen 'reflection' mean this).
'old king' becomes 'GR' (Georgius Rex, Latin for King George).
'gr' backwards is 'rg'.
'die' enclosing 'rg' is 'DIRGE'.
'to' acts as a link.
(Other definitions for dirge that I've seen before include "Dragging piece" , "Funeral hymn" , "A depressing number" , "Keen; lament" , "A slow mournful song, lament for the dead" .)