Possible Clues:

• One cracking gags — or saying it's better not to?
• Librarian's line, perhaps
• Proverb espousing the virtue of staying quiet, and a hint to a word spelled out by six letters in this puzzle
• Warning against rash talk from scold seeing line blurred?
• Ill-disposed view of din (e.g. cell's noise)?
• Unusually, GIs license ancient peace slogan
• No. 1 hit for the Tremeloes in 1967
• What good listeners know, lesson that's gone over with diligence
• Peace — or the equivalent
• Frolicking inside college in outskirts of Swindon? Best say nothing