Old Church article seen in the water (5)
I believe the answer is:
ocean
'the water' is the definition.
(ocean is a kind of water)
'old church article' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'church' becomes 'CE' (abbreviation for Church of England).
'article' becomes 'an' (indefinite article).
'o'+'ce'+'an'='OCEAN'
'seen in' acts as a link.
(Other definitions for ocean that I've seen before include "tons of water" , "Pacific, for example" , "eg Indian, Pacific" , "Salt water expanse" , "Large stretch of sea" .)