Function venue one used to sweep round completely (8)
I believe the answer is:
ballroom
'function venue' is the definition.
'one used to sweep round completely' is the wordplay.
'one used to sweep' becomes 'broom'.
'round' is an insertion indicator.
'completely' becomes 'all' ('all' can be a synonym of 'completely').
'broom' enclosing 'all' is 'BALLROOM'.
(Other definitions for ballroom that I've seen before include "Place for old-fashioned dancing - and romance?" , "In which the tango or the foxtrot are danced" , "Dance hall" , "Place for dance" , "Dance-hall" .)