Mourning becomes the legendary matricidist (7)

I believe the answer is:

electra

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'mourning' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for electra that I've seen before include "Sort of complex" , "Mythical woman with a complex?" , "heroine of Greece" , "Treacle (anag) - daughter of Agamemnon" , "legendary Greek daughter" .)

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

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