But they don't make imprisonment any smoother (5)
I believe the answer is:
irons
'smoother' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for irons that I've seen before include "Smooths, presses" , "British actor" , "for old convict to wear" , "Golf-clubs with metal heads" , "One might have too many in the fire" .)