Old article about church is deep (5)
I believe the answer is:
ocean
'is deep' is the definition.
(I've seen this in another clue)
'old article about church' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'article' becomes 'an' (indefinite article).
'about' means one lot of letters go next to another.
'church' becomes 'ce' (abbreviation for Church of England).
'an' after 'ce' is 'cean'.
'o'+'cean'='OCEAN'
(Other definitions for ocean that I've seen before include "The deep" , "Large amount of water and" , "eg Arctic" , "Large body of salt water" , "Huge sea" .)