Part of church one enters lacking sophistication (5)
I believe the answer is:
naive
'lacking sophistication' is the definition.
(I've seen this before)
'part of church one enters' is the wordplay.
'part of church' becomes 'nave' ('nave' is part of 'church').
'one' becomes 'i' (Roman numeral).
'enters' is an insertion indicator.
'nave' placed around 'i' is 'NAIVE'.
(Other definitions for naive that I've seen before include "Unaffected or guileless" , "Too trusting and inexperienced" , "Ingenous, easy to gull" , "Lacking judgment" , "Green, inexperienced" .)