Crossword Genius

Man, say, raised boy outside? It was untrue (7)

Ross

I believe the answer is:

disleal

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

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

I can't explain the rest of the clue.

Can you help me to learn more?

I've seen this clue in The Sunday Times and The Times.
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