A sheep heard in metropolis? Disaster! (8)

I believe the answer is:

calamity

I'm a little stuck... Click here to teach me more about this clue!

'disaster' is the definition.
(I know that disaster can be written as calamity)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for calamity that I've seen before include "Damaging event" , "Event resulting in great loss and misfortune" , "distress" , "Martha Jane Cannary" , "15 [JANE] the frontierswoman" .)

I've seen this clue in The Telegraph.

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