Shelling makes cruel upheaval? That’s it! (6)

I believe the answer is:

eureka

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'it' is the definition.
The definition suggests a singular noun which matches the answer.

I don't understand the rest of the clue.

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(Other definitions for eureka that I've seen before include "alloy" , "Discoverer's triumphant cry" , "Shriek of discovery" , "Exultant cry" , "Bingo!" .)

I've seen this clue in the Sydney Morning Herald.

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