Easily deceived one in part of church (5)
I believe the answer is:
naive
'easily deceived' is the definition.
'one in part of church' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'part of church' becomes 'nave' ('nave' is part of 'church').
'i' going inside 'nave' is 'NAIVE'.
(Other definitions for naive that I've seen before include "Simple in style or behaviour" , "Jejune" , "Lacking sophistication" , "Ingenous, easy to gull" , "Ingenuous, easily deceived" .)