Non-paying boarders (9)

I believe the answer is:

stowaways

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

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for stowaways that I've seen is " The hide aboard to get free passage".)

I've seen this clue in the Evening Standard.

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