Top theatre's first Evita cast to mesmerise (9)
I believe the answer is:
captivate
'to mesmerise' is the definition.
(thesaurus)
'top theatre's first evita cast' is the wordplay.
'top' becomes 'cap' (both can mean to beat or outdo).
'theatre's first' becomes 't' (1st letter of 'theatre').
'cast' indicates an anagram (letters are dispersed or cast around).
'evita' anagrammed gives 'ivate'.
'cap'+'t'+'ivate'='CAPTIVATE'
(Other definitions for captivate that I've seen before include "Enchant" , "Fascinate completely" , "Entrance" , "Delight" , "Charm" .)