Celebration when a prisoner returns (4)
I believe the answer is:
gala
'celebration' is the definition.
(gala is a kind of celebration)
'a prisoner returns' is the wordplay.
'prisoner' becomes 'lag' (I've seen this before).
'returns' shows that the letters should be reversed in order.
'a'+'lag'='alag'
'alag' back-to-front is 'GALA'.
'when' acts as a link.
(Other definitions for gala that I've seen before include "Fête" , "Carnival, fete" , "Festive celebration" , "Special social or sporting occasion" , "Special performance" .)