Dance venue (11)
'dance venue' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ballroom that I've seen before include "Dance hall" , "place associated with steps?" , "Place for old-fashioned dancing - and romance?" , "In which the tango or the foxtrot are danced" , "Dance venue" .)
(Other definitions for discotheque that I've seen before include "Dance club" , "night club" .)