A prisoner turns up at the festival (4)
I believe the answer is:
gala
'the festival' is the definition.
(thesaurus)
'a prisoner turns up' is the wordplay.
'prisoner' becomes 'lag' (I've seen this before).
'turns up' shows that the letters should be reversed in order.
'a'+'lag'='alag'
'alag' backwards is 'GALA'.
'at' is the link.
(Other definitions for gala that I've seen before include "Festival" , "A show of charity" , "Jamboree" , "Special sports event" , "Festive event" .)