Old church overlooking an expanse of water (5)
I believe the answer is:
ocean
'expanse of water' is the definition.
(I've seen this before)
'old church overlooking an' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'church' becomes 'ce' (abbreviation for Church of England).
'overlooking' means one lot of letters go next to another (down clues only).
'o'+'ce'+'an'='OCEAN'
(Other definitions for ocean that I've seen before include "A mighty sea" , "Very large area of sea" , "Large area of water" , "Vast quantity of liquid" , "Pacific, for example" .)