Navy's watch out over rocky hollow (6)
I believe the answer is:
cavern
'rocky hollow' is the definition.
'cavern' can be an answer for 'hollow' (thesaurus). I am not sure about the 'rocky' bit.
'navy's watch out over' is the wordplay.
'navy' becomes 'RN' (abbreviation for Royal Navy).
'watch out' becomes 'cave' (I've seen this before).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'rn' after 'cave' is 'CAVERN'.
Can you help me to learn more?
(Other definitions for cavern that I've seen before include "Craven in a cave" , "vast enclosed space" , "Subterranean chamber" , "Grotto" , "Large dark hole in the ground" .)