Secure area of the sea (5)
I believe the answer is:
sound
I believe this clue is a double definition.
'secure' is the first definition.
('sound' can be similar in meaning to 'secure')
'area of the sea' is the second definition.
(Other definitions for sound that I've seen before include "to do the plumbing?" , "Noise; in good condition" , "Healthy; narrow channel" , "note, for instance" , "Health - noise" .)