Generator upset many during party (6)
I believe the answer is:
dynamo
'generator' is the definition.
(I know that dynamo is a type of generator)
'upset many during party' is the wordplay.
'upset' says the letters should be written in reverse.
'during' means one lot of letters goes inside another.
'party' becomes 'do' (do is a kind of party).
'many' backwards is 'ynam'.
'ynam' inserted inside 'do' is 'DYNAMO'.
(Other definitions for dynamo that I've seen before include "Source of electric current" , "Producer of electrical energy" , "Energy converter - hard worker" , "Forceful person" , "Generator (sometimes 'human'?)" .)