Possible Clues:

• Cold enough to turn to an unaspirated, hard case
• The cockney comes back as a hard nut from the 21 across
• A cold, hard, unaspirated nut up there
• Treeless northern region
• `Ard thing to crack up when it's frozen
• Plain fish contains bones
• It's hard underneath for the turn of an unaspirated hard nut
• Underneath it's hard enough to make it a hard nut for the unaspirated
• Beat it and dial it at last
• Here it's cold fish, mostly dry inside