Northerly wind captured in photo of the open sea (7)
I believe the answer is:
pelagic
'of the open sea' is the definition.
(pelagic means relating to the sea)
'northerly wind captured in photo' is the wordplay.
'northerly' says the letters should be written backwards (in a down clue, letters go up or 'north').
'wind' becomes 'gale' (gale is a kind of wind).
'captured in' is an insertion indicator.
'photo' becomes 'pic' (short for picture).
'gale' backwards is 'elag'.
'elag' put within 'pic' is 'PELAGIC'.
(Other definitions for pelagic that I've seen before include "of the ocean" , "Marine" , "Sea" , "Inhabiting the deep" , "Like an albatross" .)