Bishop organised church's regional office (6)
I believe the answer is:
branch
'regional office' is the definition.
(I've seen this before)
'bishop organised church's' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'organised' becomes 'ran' (run can mean to organise or lead).
'church' becomes 'ch' (common abbreviation).
'b'+'ran'+'ch'='BRANCH'
(Other definitions for branch that I've seen before include "Bough - offshoot - secondary office" , "Tree limb" , "Lateral extension of river or railway" , "'Fork, offshoot (6)'" , "Part of organisation or tree" .)