Crossword Genius

Oxbridge competitor unhappy (4)

Ross

I believe the answer is:

blue

I'm a little stuck... Click here to teach me more about this clue!

This might be a double definition.

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

'unhappy' is the second definition.
(blue can mean sad or miserable)

Can you help me to learn more?

(Other definitions for blue that I've seen before include "Dejected" , "Such humour < makes conservative" , "Sky-coloured" , "coarse" , "Unsuitable for young" .)

I've seen this clue in The Guardian.
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