Let out on reversal of prison sentence (4)
I believe the answer is:
emit
'let out' is the definition.
(I've seen this in another clue)
'reversal of prison sentence' is the wordplay.
'reversal of' shows that the letters should be reversed in order.
'prison sentence' becomes 'time' (as in 'doing time').
'time' reversed gives 'EMIT'.
'on' is the link.
(Other definitions for emit that I've seen before include "'Give off, odour say (4)'" , "Small turtle" , "Issue" , "shed" , "Give off, heat say" .)