Gullible individual’s initiation in church body (5)
I believe the answer is:
naive
'gullible' is the definition.
(similar in meaning)
'individual's initiation in church body' is the wordplay.
'initiation' says to take the initial letters.
'in' is an insertion indicator.
'church body' becomes 'nave' (part of a church building).
The initial letter of 'individual' is 'i'.
'i' placed inside 'nave' is 'NAIVE'.
(Other definitions for naive that I've seen before include "Without worldly wisdom" , "Simple in outlook" , "Excessively trusting or innocent" , "Inexperienced, innocent" , "Ingenuous and credulous" .)