A delay over celebration (4)
I believe the answer is:
gala
'celebration' is the definition.
(gala is a kind of celebration)
'a delay over' is the wordplay.
'delay' becomes 'lag' (lag is a kind of delay).
'over' 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 "A festive occasion, perhaps of sporting nature" , "Sports meeting" , "A show of charity" , "Carnival, fete" , "Carnival, celebration" .)