Festivity has a prisoner turning up (4)
I believe the answer is:
gala
'festivity' is the definition.
(gala is a kind of festivity)
'a prisoner turning up' is the wordplay.
'prisoner' becomes 'lag' (slang term).
'turning up' is a reversal indicator.
'a'+'lag'='alag'
'alag' backwards is 'GALA'.
'has' acts as a link.
(Other definitions for gala that I've seen before include "Mix" , "Jamboree" , "Special social or sporting occasion" , "Fête" , "Special sports event" .)