Credulous one visiting part of church (5)
I believe the answer is:
naive
'credulous' is the definition.
(associated in meaning)
'one visiting part of church' is the wordplay.
'one' becomes 'i' (Roman numeral).
'visiting' is an insertion indicator.
'part of church' becomes 'nave' (area of a church building).
'i' inserted into 'nave' is 'NAIVE'.
(Other definitions for naive that I've seen before include "Gullible, too trusting" , "childlike" , "Unaffected or guileless" , "Dewy-eyed" , "Too trusting and impractical" .)