Innocent one is found in part of the church (5)
I believe the answer is:
naive
'innocent' is the definition.
('naive' can be similar in meaning to 'innocent')
'one is found in part of the church' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is found in' is an insertion indicator.
'part of the church' becomes 'nave' ('nave' is part of 'church').
'i' going into 'nave' is 'NAIVE'.
(Other definitions for naive that I've seen before include "Unaffected or guileless" , "Innocently trustful" , "Foolish, inexperienced" , "Callow" , "Inexperienced, innocent" .)