Depressingly indecent? (4)
I believe the answer is:
blue
'indecent?' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for blue that I've seen before include "Turquoise, say" , "over-extravagant" , "Glum, sad" , "Unsuitable for young" , "Down in the mouth" .)