Extremely unnatural being upset over party game (4)
I believe the answer is:
ludo
'game' is the definition.
(ludo is a board game)
'extremely unnatural being upset over party' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'being upset over' shows that the letters should be reversed in order (knocked over).
'party' becomes 'do' (do can informally mean a party).
'unnatural' with its centre taken out is 'ul'.
'ul' written backwards gives 'lu'.
'lu'+'do'='LUDO'
(Other definitions for ludo that I've seen before include "Simple board game, involving dice" , "Game (meaning 'I play' in Latin)" , "Loud sort of board game with dice and counters" , "Traditional board game" , "race for four tokens" .)