Crossword Genius

Moody policeman (4)

Ross

I believe the answer is:

blue

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This clue could be a double definition.

'moody' is the first definition.
(thesaurus)

'policeman' is the second definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

Can you help me to learn more?

(Other definitions for blue that I've seen before include "Dejected" , "Down in the mouth" , "Dirty" , "Such humour < makes conservative" , "Spend all at once - colour" .)

I've seen this clue in The Times.
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