Innocent one in part of church (5)
I believe the answer is:
naive
'innocent' is the definition.
(similar in meaning)
'one in part of church' is the wordplay.
'one' becomes 'I' (Roman numeral).
'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 "Too simple and trusting" , "Inexperienced and guileless" , "childlike" , "Unaffected or guileless" , "Unaffected; credulous" .)