Turn to put out what prisoners do (4)
I believe the answer is:
time
'what prisoners do' is the definition.
(I've seen this in another clue)
'turn to put out' is the wordplay.
'turn' says the letters should be written in reverse.
'to put out' becomes 'emit' (I've seen this in another clue).
'emit' backwards is 'TIME'.
(Other definitions for time that I've seen before include "Prison sentence; tempo" , "Its detonation is delayed" , "See 1 and 21" , "Hours clocked up?" , "Age" .)