Short musical drama (8)
I believe the answer is:
operetta
'short musical drama' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for operetta that I've seen before include "Light opera" , "The Mikado or The Gondoliers, say" , "The Merry Widow, for example" , "Short amusing opera" , "Type of musical drama" .)