Crossword Genius

One is all in when such a portrait is completed (4,6)

Ross

I believe the answer is:

full length

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'one' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I don't see?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for full length that I've seen is " covering entire distance".)

I've seen this clue in The Globe and Mail.
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