A delay put back festivity … (4)
I believe the answer is:
gala
'festivity' is the definition.
(gala is a kind of festivity)
'a delay put back' is the wordplay.
'delay' becomes 'lag' (lag is a kind of delay).
'put back' says the letters should be written backwards.
'a'+'lag'='alag'
'alag' written backwards gives 'GALA'.
(Other definitions for gala that I've seen before include "Mix" , "Public festivity" , "Special performance" , "Social occasion with special performances" , "Special social or sporting occasion" .)