Crossword Genius

Kid's father? (5,4)

Ross

I believe the answer is:

billy goat

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

This clue could be a double definition.

'kid's' is the first definition.
(thesaurus)

'father?' is the second definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

Can you help me to learn more?

I've seen this clue in The Telegraph.
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