One-time conference venue set up by volunteers (5)
I believe the answer is:
yalta
'one-time conference venue' is the definition.
(as in the Yalta conference of 1945)
'set up by volunteers' is the wordplay.
'set' becomes 'lay' (both can mean to place something).
'up by' says the letters should be written backwards.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'lay' written backwards gives 'yal'.
'yal'+'ta'='YALTA'
(Other definitions for yalta that I've seen before include "Crimean town famous for WW2 conference" , "Black Sea resort" , "Black Sea port" , "Site of the Allied conference in 1945" , "Crimean resort city" .)