Related Clues:

• The word, as suggested by the saying formed by the ends of this puzzle's four longest answers
• On the q.t
• Mum, listen for a change!
• Listen anyhow, without a sound
• Imprison youth leader for being cautious
• The wallflower is, with nitre etc scattered around
• During break, call back mum
• I act dumb and act tight-lipped
• Dumb boy in post
• Dumb as the majority are reported to be?
• Dumb animal turns; then I turn
• Pioneering ecological book is back, fast bound
• Film gangster loses point, being cautious
• Understood vessel can be reserved
• I convert after dreadful act — that's not saying much