City retreat upset churchman (6)
I believe the answer is:
denver
'city' is the definition.
(Denver is an example)
'retreat upset churchman' is the wordplay.
'retreat' becomes 'den' (both can mean a lair or hideout).
'upset' is a reversal indicator.
'churchman' becomes 'rev' (abbreviation for Reverend).
'rev' backwards is 'ver'.
'den'+'ver'='DENVER'
(Other definitions for denver that I've seen before include "US state capital" , "Capital of Colorado state" , "Capital of Colorado, USA" , "Country singer and Colorado city" , "US city" .)