Step taken by one in prison? (5)
I believe the answer is:
stair
'step' is the definition.
(a single step in a staircase)
'one in prison?' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'in' means one lot of letters goes inside another.
'prison?' becomes 'stir' (informal term for prison).
'a' inserted into 'stir' is 'STAIR'.
'taken by' acts as a link.
(Other definitions for stair that I've seen before include "Support on which to step" , "Set of steps" , "that's between levels" , "One of a 15" , "Rising step" .)