Person who criticises prison? (7)
I believe the answer is:
slammer
'prison?' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for slammer that I've seen before include "Stir" , "one leaping into crowd" , "He noisily leaves the room" , "One pulling off lots of tricks" , "One severely criticises" .)