Beware hollow chamber (4)
I believe the answer is:
cave
I believe this clue is a triple definition!
'beware' is the first definition.
(old-fashioned word meaning 'beware')
'hollow' is the second definition.
(caving is a kind of hollowing)
'chamber' is the third definition.
(I have seen 'Underground chamber' mean 'cave' so perhaps 'chamber' could also mean 'cave')
(Other definitions for cave that I've seen before include "Hollow in a rock" , "Natural hollow in side of hill or cliff" , "Natural underground space" , "Primitive dwelling" , "Watch out" .)