Water area covered by turf (4)
I believe the answer is:
soda
'water' is the definition.
(soda water is a kind of water)
'area covered by turf' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'covered by' is a charade indicator (letters next to each other) (on top of in a down clue).
'turf' becomes 'sod' (both can mean a piece of grass).
'a' put after 'sod' is 'SODA'.
(Other definitions for soda that I've seen before include "In the US, a sweet fizzy drink" , "Whisky mixer" , "pressurised liquid" , "Type of bread, water or fountain" , "Accompaniment to whisky" .)