Showing an innocence one found in large part of church (5)
I believe the answer is:
naive
'showing an innocence' is the definition.
(naive can mean innocent)
'one found in large part of church' is the wordplay.
'one' becomes 'i' (Roman numeral).
'found in' indicates putting letters inside.
'large part of church' becomes 'nave' (part of a church building).
'i' inserted inside 'nave' is 'NAIVE'.
(Other definitions for naive that I've seen before include "Callow" , "Rather too simple and gullible" , "Lacking judgment" , "Inexperienced, innocent" , "Wet behind ears" .)