Theatrical entertainment overlong? Switch each half around (5)
I believe the answer is:
panto
'theatrical entertainment' is the definition.
(short for pantomime)
'overlong? switch each half around' is the wordplay.
'overlong? switch each half' becomes 'pant' (I can't explain this - if you can you should believe this answer much more).
'around' becomes 'o' (I've seen this in other clues).
'pant'+'o'='PANTO'
(Other definitions for panto that I've seen before include "festive entertainment" , "lowbrow entertainment" , "Traditional seasonal entertainment (abbr)" , "cross-dressing performance" , "Dick Whittington" .)