Jack loses his head in part of church (4)
I believe the answer is:
nave
'part of church' is the definition.
(area of a church building)
'jack loses his head' is the wordplay.
'jack' becomes 'knave' (knave can mean a jack playing card).
'loses his head' means to remove the first letter.
'knave' with its initial letter taken away is 'NAVE'.
'in' acts as a link.
(Other definitions for nave that I've seen before include "Part of the church sounds like a rogue" , "Take no risks" , "Middle of wheel" , "Central space in church" , "The hub of a wheel, or body of church" .)