The most common and most likely answer to this crossword clue is the 7 letter word SLAMMER
I've seen this crossword clue in The Daily Telegraph
S
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More clues leading to the result SLAMMER
Buddy explains!
The straight part
We know that "One who criticises/prison" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one who criticises/prison = slammer.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SLAMMER mean?
SLAMMER noun- One who, or that which, slams.
- (usually "the slammer") Jail, prison.
- A tequila cocktail.
- One who takes part in slam-dance.
- One who competes in a poetry slam.
- In the game of Pogs, the heavier piece used to strike the stack of counters.
- A slam-door train.