Upset, pasty-faced PC leaves festival (5,3)
I believe the answer is:
feast day
'festival' is the definition.
(I've seen this before)
'upset pasty-faced pc leaves' is the wordplay.
'upset' indicates an anagram.
'leaves' indicates named letters should be taken away.
'pasty'+'faced'='pastyfaced'
'pastyfaced' with 'pc' taken away is 'astyfaed'.
'astyfaed' with letters rearranged gives 'FEAST-DAY'.
(Other definitions for feast day that I've seen before include "Religious celebration" , "Church holiday for eating and rejoicing - fed satay (anag)" , "Annual Christian celebration" , "time to celebrate" , "festival" .)