Thank you Thea for your response, combining equinity and divinity. Despite the absolute authority, I'm still uncomfortable with the clue as I don't find 'rent/rend' synonymous with 'crack'. 'Crack' relates to brittle or frangible substances, 'rend/rent' to fabrics or other tractable/malleable materials. But there you are -'keep calm..,' etc.
Incidentally, from the introduction of your response, do I detect that my alias is beginning to unravel?