Crossword Genius

Rugger to be important? (6)

Ross

I believe the answer is:

matter

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'rugger' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for matter that I've seen before include "Substance as opposed to spirit" , "Stuff - be of importance" , "Substance; topic" , "From which all physical things are made" , "Subject; be important" .)

I've seen this clue in The Sun.
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