Beware sailors in Beatles' club (6)
I believe the answer is:
cavern
'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" .)