Mixed rum cup: it’s so delightful! (11)
I believe the answer is:
scrumptious
'delightful' is the definition.
(thesaurus)
'mixed rum cup it's so' is the wordplay.
'mixed' indicates an anagram.
'rum'+'cup'+'its'+'so'='rumcupitsso'
'rumcupitsso' anagrammed gives 'SCRUMPTIOUS'.
(Other definitions for scrumptious that I've seen before include "Yummy" , "Toothsome" , "very good" , "Extremely appetising (colloq.)" , "Delightful" .)