In a blue mood (3)
I believe the answer is:
sad
'in a blue mood' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sad that I've seen before include "Sorrowful, unhappy" , "Despondent" , "Mournful and depressed" , "Mournful - sorry" , "grey" .)