Mourning becomes the legendary matricidist (7)
I believe the answer is:
electra
'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" .)