Innocent I has divided part of church (5)
I believe the answer is:
naive
'innocent' is the definition.
(similar in meaning)
'i has divided part of church' is the wordplay.
'has divided' means one lot of letters goes inside another.
'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 "Credulous, unworldly" , "Ingenuous and gullible" , "Rather too simple and ingenuous" , "Inexperienced or gullible" , "Ingenuous, easily deceived" .)