Release when prison sentence is over (4)
I believe the answer is:
emit
'release' is the definition.
(to emit is to release or let out)
'prison sentence is over' is the wordplay.
'prison sentence' becomes 'time' (as in 'doing time').
'is over' says the letters should be written backwards.
'time' backwards is 'EMIT'.
'when' acts as a link.
(Other definitions for emit that I've seen before include "'Give off, odour say (4)'" , "Make audible sounds" , "'Give out, light say (4)'" , "Sent" , "Small turtle" .)