Step out of a prison (4)
I believe the answer is:
stir
'prison' is the definition.
(informal term for prison)
'step out of a' is the wordplay.
'step' becomes 'stair' (a single step in a staircase).
'out of' suggests deleting specific letters.
'stair' with 'a' taken away is 'STIR'.
(Other definitions for stir that I've seen before include "Sensation" , "Rotate spoon" , "Awaken" , "Slight movement" , "Disturb" .)