A delay over festival (4)
I believe the answer is:
gala
'festival' is the definition.
(a gala is a festival or party)
'a delay over' is the wordplay.
'delay' becomes 'lag' (lag is a kind of delay).
'over' shows that the letters should be reversed in order.
'a'+'lag'='alag'
'alag' written backwards gives 'GALA'.
(Other definitions for gala that I've seen before include "Jamboree" , "Festival" , "Festive event" , "Special social or sporting occasion" , "field day" .)