One in a church body is innocent (5)
I believe the answer is:
naive
'innocent' is the definition.
(similar in meaning)
'one in a church body' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'a church body' becomes 'nave' (the nave is the main part or body of a church).
'i' placed inside 'nave' is 'NAIVE'.
'is' is the link.
(Other definitions for naive that I've seen before include "Rather too simple and ingenuous" , "Too simple and trusting" , "Inexperienced, gullable" , "Showing lack of maturity or judgement" , "Unsophisticated or inexperienced" .)