Beware rocky hollow (4)
I believe the answer is:
cave
This could be a double definition.
'beware' is the first definition.
(old-fashioned term meaning 'look out')
'rocky hollow' is the second definition.
'cave' can be an answer for 'hollow' (I've seen this in another clue). I am not certain of the 'rocky' bit.
Can you help me to learn more?
(Other definitions for cave that I've seen before include "Hollow - warning" , "Natural underground space" , "Warning - grotto" , "Hollow in a rock" , "part of rocky coastline?" .)