Devise an opening at sea (5)
I believe the answer is:
hatch
This clue could be a double definition.
'devise an' is the first definition.
'hatch' can be an answer for 'devise' (as in hatching a plan). I'm not certain of the 'an' bit.
'opening at sea' is the second definition.
'hatch' can be an answer for 'opening' (I've seen this before). I'm not certain of the 'at sea' bit.
Can you help me to learn more?
(Other definitions for hatch that I've seen before include "Enter the world" , "contrive to > emerge in > shade" , "Of young bird or fish, emerge from egg" , "Opening in floor or wall" , "Deck opening; (egg) open" .)