Fellows with long underpants? (5)

I believe the answer is:

johns

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'fellows' is the definition.
The answer and definition can be both people as well as being plural nouns.
Perhaps you can see a link between them that I don't see?

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

Can you help me to learn more?

(Another definition for johns that I've seen is " man behind Biggles".)

I've seen this clue in the Evening Standard.

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