Young goat is an outlaw (5,3,3)

I believe the answer is:

billy the kid

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'young' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'goat is an outlaw' is the wordplay.
I cannot quite see how this works, but
'goat' could be 'billy' (billy is a kind of goat) and 'billy' is located in the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Mirror.

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