Crossword Genius

This marine is a blue (5)

Ross

I believe the answer is:

ultra

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This could be a double definition.

'this marine' is the first definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?

'blue' is the second definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?

'is a' is the link.

Can you help me to learn more?

(Other definitions for ultra that I've seen before include "Prefix meaning excessively or extremely" , "Saharan dweller" , "Project started at Bletchley Park in 1939 to break the Enigma code" , "Extreme - extremist" , "(One who is) extreme (in eg political opinion)" .)

I've seen this clue in The Guardian.
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