Blue mattress filler? (4)
I believe the answer is:
down
This could be a double definition.
'blue' is the first definition.
(both can mean sad)
'mattress filler?' is the second definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps there's an association between them I don't understand?
Can you help me to learn more?
(Other definitions for down that I've seen before include "Pillow filler - dejected" , "Out of sorts" , "Unhappy; soft feathers" , "Depressed - feathers" , "See 18" .)