Cushion’s blue under raised edge (6)
I believe the answer is:
pillow
'cushion's' is the definition.
(pillow is a kind of cushion)
'blue under raised edge' is the wordplay.
'blue' becomes 'low' (both can mean sad).
'under' means one lot of letters go next to another (below, in a down clue).
'raised' says the letters should be written backwards (going up in a down clue).
'edge' becomes 'lip' (lip is a kind of edge).
'lip' in reverse letter order is 'pil'.
'low' after 'pil' is 'PILLOW'.
(Other definitions for pillow that I've seen before include "Bolster, perhaps" , "Cushion for head" , "Bed cushion" , "Sleeper's support" , "Head supporter in bed" .)