A delay upsets the festival (4)
I believe the answer is:
gala
'the festival' is the definition.
(a gala is a festival or party)
'a delay upsets' is the wordplay.
'delay' becomes 'lag' (lag is a kind of delay).
'upsets' says the letters should be written in reverse.
'a'+'lag'='alag'
'alag' backwards is 'GALA'.
(Other definitions for gala that I've seen before include "Public festivity" , "Festival" , "A festive occasion, perhaps of sporting nature" , "Festive performance" , "Fête" .)