Tone down (4)
I believe the answer is:
blue
This clue could be a double definition.
'tone' is the first definition.
The answer and definition can be both human attributes as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'down' is the second definition.
(both can mean sad)
Can you help me to learn more?
(Other definitions for blue that I've seen before include "Colour of depression" , "Sad; university sportsman" , "Colour of low spirits" , "Turquoise, say" , "Oxbridge player" .)