Church body a Welshman set up (4)
I believe the answer is:
nave
'church body' is the definition.
(the nave is the main part or body of a church)
'a welshman set up' is the wordplay.
'a welshman' becomes 'Evan' (common Welsh name).
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'evan' reversed gives 'NAVE'.
(Other definitions for nave that I've seen before include "Central part of church" , "Central space in church" , "Main body of a church" , "Part of the church sounds like a rogue" , "Central area in a church" .)