It's even more lacklustre: that's what's wrong (6)

I believe the answer is:

matter

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'wrong' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I don't understand the rest of the clue.

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(Other definitions for matter that I've seen before include "Physicist giving name to frequency shift" , "That which occupies space in the visible world" , "Substance or material - not spirit" , "Affair; be important" , "Stuff - mean something" .)

I've seen this clue in the Evening Standard.

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