A convict returning for festival (4)
I believe the answer is:
gala
'for festival' is the definition.
(a gala is a festival or party)
'a convict returning' is the wordplay.
'convict' becomes 'lag' (slang term for a prisoner).
'returning' is a reversal indicator.
'a'+'lag'='alag'
'alag' in reverse letter order is 'GALA'.
(Other definitions for gala that I've seen before include "Carnival, fete" , "Festive occasion - variety of apple" , "Prod with the elbow" , "A festive occasion, perhaps of sporting nature" , "Festive event" .)