Singular step for one in prison (5)
I believe the answer is:
stair
'singular step' is the definition.
'stair' can be an answer for 'step' (synonyms). I'm not certain of the 'singular' bit.
'one in prison' is the wordplay.
'one' becomes 'a'.
'in' means one lot of letters goes inside another.
'prison' becomes 'stir' (informal term for prison).
'a' going inside 'stir' is 'STAIR'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for stair that I've seen before include "single step in a series" , "Step as part of flight" , "Flight unit" , "A step on the way up or down" , "Tarsi (anag)" .)