Born yesterday -- this person in main part of church (5)
I believe the answer is:
naive
'born yesterday' is the definition.
(I've seen this in another clue)
'this person in main part of church' is the wordplay.
'this person' becomes 'I'.
'in' means one lot of letters goes inside another.
'main part of church' becomes 'nave' (I've seen this before).
'i' put inside 'nave' is 'NAIVE'.
(Other definitions for naive that I've seen before include "Lacking sophistication" , "Callow" , "Lack of experience, innocent" , "Innocent and credulous" , "Lacking judgment" .)