Work, eastern treat that’s funny — The Mikado? (8)
I believe the answer is:
operetta
'the mikado?' is the definition.
(The Mikado is an example of an operetta)
'work eastern treat that's funny' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'eastern' becomes 'e'.
'that's funny' indicates anagramming the letters.
'treat' with letters rearranged gives 'retta'.
'op'+'e'+'retta'='OPERETTA'
(Other definitions for operetta that I've seen before include "Type of musical drama" , "Singing" , "light-hearted performance" , "The Mikado, e.g." , "Short musical drama" .)