Main circus tent (3,3)
'main circus tent' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for big top that I've seen before include "Circus venue" , "'Main circus tent (3,3)'" , "performance in The Ring" , "entertainment centre" , "needed by Mae West, for one?" .)
(Another definition for bigtop that I've seen is " tent".)