One entering part of the church, innocent (5)
I believe the answer is:
naive
'innocent' is the definition.
('naive' can be similar in meaning to 'innocent')
'one entering part of the church' is the wordplay.
'one' becomes 'i' (Roman numeral).
'entering' indicates putting letters inside.
'part of the church' becomes 'nave' ('nave' is part of 'church').
'i' put into 'nave' is 'NAIVE'.
(Other definitions for naive that I've seen before include "Rather too simple and ingenuous" , "Unenlightened" , "Lacking guile" , "Lacking judgment" , "Too simple and trusting" .)