One severely criticising prison (7)
I believe the answer is:
slammer
'prison' is the definition.
(I know that slammer is a type of prison)
'one severely criticising' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'me' (a setter might use 'one' to humourously mean 'me') and 'me' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for slammer that I've seen before include "Pickwickian doctor" , "Prison (slang)" , "He noisily leaves the room" , "Prison (sl.)" , "one leaping into crowd" .)