Water may flow through this land in the sea (5)
I believe the answer is:
ditch
I believe this clue is a double definition.
'water may flow through this' is the first definition.
'land in the sea' is the second definition.
(to ditch an aircraft is to land it on water)
(Other definitions for ditch that I've seen before include "Trench, dike" , "Scrap" , "Drop" , "Drainage trench" , "Gully or furrow" .)