Vast body of water (5)
I believe the answer is:
ocean
'vast body of water' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ocean that I've seen before include "Large body of salt water" , "An immense expanse of sea" , "Huge body of water" , "eg Indian, Atlantic" , "Quantity of water" .)