Upset a rum at party (6)
I believe the answer is:
trauma
'upset' is the definition.
(thesaurus)
'a rum at party' is the wordplay.
'party' is an anagram indicator (I've seen 'parties' mean this (the letters move or party around)).
'a'+'rum'+'at'='arumat'
'arumat' is an anagram of 'TRAUMA'.
(Other definitions for trauma that I've seen before include "Physical or mental shock" , "Nervous shock" , "Psychological wound" , "dreadful shock!" , "Bodily or mental shock" .)