Fun to tie cast in knots — that's entertainment (4,3,5)
I believe the answer is:
cosi fan tutte
'entertainment' is the definition.
(Cos fan tutte is an opera by Mozart)
'fun to tie cast in knots' is the wordplay.
'in knots' indicates anagramming the letters (I've seen this in other clues).
'fun'+'to'+'tie'+'cast'='funtotiecast'
'funtotiecast' anagrammed gives 'COSI FAN TUTTE'.
'that's' acts as a link.
(Other definitions for cosi fan tutte that I've seen before include "One of Mozart's 'da Ponte' operas" , "Mozart opera" .)