You wouldn't call one "old scout" (3)

I believe the answer is:

cub

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'scout' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe there's an association between them I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for cub that I've seen before include "Young bear or lion" , "Junior scout; puppy" , "A baby fox or bear" , "eg Young lion" , "Young one" .)

I've seen this clue in the Evening Standard.

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