Crossword Genius

Beware sailors in Beatles' club (6)

Ross

I believe the answer is:

cavern

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

'beatles' club' is the definition.
I can't judge whether this defines the answer.

'beware sailors' is the wordplay.
'beware' becomes 'cave' (old-fashioned word meaning 'beware').
'sailors' becomes 'RN' (Royal Navy).
'cave'+'rn'='CAVERN'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for cavern that I've seen before include "Large underground hollow" , "vast enclosed space" , "Large dark hole in the ground" , "Liverpool club where The Beatles started out" , "Subterranean chamber" .)

I've seen this clue in The Sun.
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