Crossword Genius

One brought up to be unchangingly paternal? (3)

Ross

I believe the answer is:

dad

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'one' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

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

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(Other definitions for dad that I've seen before include "Popular and informal father" , "Mum's mate" , "Papa" , "Pa" , "Popular father - a palindrome" .)

I've seen this clue in the Evening Standard.
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