A shade low (4)
I believe the answer is:
blue
This clue is a double definition.
'a shade' is the first definition.
(I've seen this before)
'low' is the second definition.
(both can mean unhappy)
(Other definitions for blue that I've seen before include "Naughty < sportsman" , "Colour - smutty" , "Sad - shade" , "Turquoise, say" , "Unhappy a sportsman at university" .)