A convict turned up for the festivity (4)
I believe the answer is:
gala
'the festivity' is the definition.
(gala is a kind of festivity)
'a convict turned up' is the wordplay.
'convict' becomes 'lag' (I've seen this before).
'turned up' shows that the letters should be reversed in order.
'a'+'lag'='alag'
'alag' in reverse letter order is 'GALA'.
'for' is the link.
(Other definitions for gala that I've seen before include "Swimming competition" , "Festive entertainment" , "A show of charity" , "Festive event" , "Fête" .)