Opera with spoken dialogue, one mostly including a lot of agents (9)
I believe the answer is:
singspiel
'opera with spoken dialogue' is the definition.
(type of German opera)
'one mostly including a lot of agents' is the wordplay.
'one' becomes 'single' (synonyms).
'mostly' means to remove the last letter.
'including' means one lot of letters goes inside another.
'a lot of agents' becomes 'spie' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'single' with its final letter taken away is 'singl'.
'singl' enclosing 'spie' is 'SINGSPIEL'.
(Another definition for singspiel that I've seen is " opera".)