Dismal party game is upsetting (4)
I believe the answer is:
dour
'dismal' is the definition.
(thesaurus)
'party game is upsetting' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'game' becomes 'ru' (rugby union).
'is upsetting' is a reversal indicator.
'ru' in reverse letter order is 'ur'.
'do'+'ur'='DOUR'
(Other definitions for dour that I've seen before include "Erratic" , "Gloomy-looking, stern" , "'Sullen, sour (4)'" , "Obstinate - grim" , "Constituent parts" .)