Party's upset by this food (6)
I believe the answer is:
pastry
'this food' is the definition.
(pastry is a kind of food)
'party's upset' is the wordplay.
'upset' is an anagram indicator.
'partys' is an anagram of 'PASTRY'.
'by' acts as a link.
(Other definitions for pastry that I've seen before include "Cake - tart" , "Dough made for pies, etc." , "Pie base" , "'It may be puff, shortcrust and flakey (6)'" , "Baker's product" .)