Eyot; cay (4)
I believe the answer is:
isle
This clue is a double definition.
'eyot' is the first definition.
(I've seen this before)
'cay' is the second definition.
(I've seen this in another clue)
(Other definitions for isle that I've seen before include "Piece of land in water" , "being impossible without water" , "Ait" , "Lies (anag)" , "This could be key" .)