Possible Clues:

• Castle cell
• Keep order to pay £1,000 a long time
• Keep manure working round top of earth
• Horse has gone off to get cooler
• Unpleasant cell
• Muck over time found in subterraneous accommodation
• Mafia boss must stifle limitless appetite for prison!
• Was tunnelling long time to get outside new prison
• Grey, say, up above, cooler underground
• Debt-collector gone mad in underground cell