A convict turns up for a festive occasion (4)
I believe the answer is:
gala
'festive occasion' is the definition.
(thesaurus)
'a convict turns up for a' is the wordplay.
'a convict' becomes 'lag' (I've seen this before).
'turns up' says the letters should be written backwards.
'for' is a charade indicator (letters next to each other).
'lag' in reverse letter order is 'gal'.
'gal'+'a'='GALA'
(Other definitions for gala that I've seen before include "Carnival, celebration" , "Mix" , "A show of charity" , "Festive occasion - variety of apple" , "Jamboree" .)