Energetic person upset a large number in party (6)
I believe the answer is:
dynamo
'energetic person' is the definition.
(I've seen this in another clue)
'upset a large number in party' is the wordplay.
'upset' shows that the letters should be reversed in order.
'a large number' becomes 'many' (many means lots of something).
'in' means one lot of letters goes inside another.
'party' becomes 'do' (do can informally mean a party).
'many' back-to-front is 'ynam'.
'ynam' put into 'do' is 'DYNAMO'.
(Other definitions for dynamo that I've seen before include "Generator of electricity (eg, on bicycle)" , "Type of electrical generator" , "Electric current generator" , "Electric current producer" , "Device producing electrical power" .)