Clue: Say, borstal pupil's denial retracted after pressure? (6)
Answer: PRISON
Explanation: Definition: Say, borstal
 **Reversal of**(… retracted) [ NO, SIR ](a pupils' denial as said to a teacher) **placed after**(after) P(symbol for "pressure" in physics).
Defn: An example of which/say, is the borstal, a custodial institution for young offenders.