Beware! Large hole underground (4)
I believe the answer is:
cave
This clue is a double definition.
'beware' is the first definition.
(old-fashioned term meaning 'look out')
'large hole underground' is the second definition.
(Other definitions for cave that I've seen before include "Natural subterranean opening" , "Large grotto" , "Underground enclosure with surface access" , "Beware" , "Hollow in a rock" .)