Innocent the First dividing part of church (5)
I believe the answer is:
naive
'innocent' is the definition.
(similar in meaning)
'first dividing part of church' is the wordplay.
'first' becomes 'i' (Roman numeral for one or first).
'dividing' indicates putting letters inside (inserted letters divide the word).
'part of church' becomes 'nave' (area of a church building).
'i' placed within 'nave' is 'NAIVE'.
'the' is the link.
(Other definitions for naive that I've seen before include "Simple in style or behaviour" , "Inexperienced or gullible" , "Short on experience" , "Too simple and trusting" , "Simple in outlook" .)