Part of flight from a prison's over (5)
I believe the answer is:
stair
'part of flight' is the definition.
(I've seen this before)
'a prison's over' is the wordplay.
'prison' becomes 'stir' (informal term for prison).
'over' indicates putting letters inside.
'a' inserted inside 'stir' is 'STAIR'.
'from' acts as a link.
(Other definitions for stair that I've seen before include "A step on the way up or down" , "One step of a flight" , "Tread and riser" , "Bit of a flight!" , "Step in house" .)