Beware of sailors in underground chamber (6)
I believe the answer is:
cavern
'underground chamber' is the definition.
(I've seen this before)
'beware of sailors' is the wordplay.
'beware' becomes 'cave' (old-fashioned term meaning 'look out').
'of sailors' becomes 'rn' (I can't justify this - if you can you should believe this answer much more).
'cave'+'rn'='CAVERN'
'in' acts as a link.
(Other definitions for cavern that I've seen before include "Liverpool club where The Beatles started out" , "Large underground hollow" , "Craven in a cave" , "Grotto" , "Subterranean chamber" .)