Upset at charges, enter uninvited (9)
I believe the answer is:
gatecrash
'enter uninvited' is the definition.
'upset at charges' is the wordplay.
'upset' indicates anagramming the letters.
'at'+'charges'='atcharges'
'atcharges' is an anagram of 'GATECRASH'.
(Other definitions for gatecrash that I've seen before include "enter without invitation" , "to do so uninvited" , "Try to get in for free" , "join party unofficially" , "join party if not wanted" .)