Innocent one in church body (5)
I believe the answer is:
naive
'innocent' is the definition.
('naive' can be similar in meaning to 'innocent')
'one in church body' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'church body' becomes 'nave' (I've seen this in other clues).
'i' inserted inside 'nave' is 'NAIVE'.
(Other definitions for naive that I've seen before include "Ingenuous, easily deceived" , "Gullible, too trusting" , "Unaffected; credulous" , "Rather too simple and gullible" , "Ingenuous and gullible" .)