Innocent one hiding in part of church (5)
I believe the answer is:
naive
'innocent' is the definition.
('naive' can be similar in meaning to 'innocent')
'one hiding in part of church' is the wordplay.
'one' becomes 'i' (Roman numeral).
'hiding in' indicates putting letters inside.
'part of church' becomes 'nave' ('nave' is part of 'church').
'i' going into 'nave' is 'NAIVE'.
(Other definitions for naive that I've seen before include "Simple in style or behaviour" , "Lacking guile" , "Rather too simple and ingenuous" , "Lack of experience, innocent" , "Short on experience" .)