Electricity provider upset a lot in party (6)
I believe the answer is:
dynamo
'electricity provider' is the definition.
(type of generator)
'upset a lot in party' is the wordplay.
'upset' says the letters should be written backwards.
'a lot' becomes 'many' ().
'in' means one lot of letters goes inside another.
'party' becomes 'do' (do is a kind of party).
'many' back-to-front is 'ynam'.
'ynam' going within 'do' is 'DYNAMO'.
(Other definitions for dynamo that I've seen before include "One creating electricity" , "Energy converter - hard worker" , "something electrifying" , "activist?" , "Power source" .)