Importance of mating due to be re-evaluated (9)

I believe the answer is:

magnitude

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'importance' is the definition.
(I know that magnitude is a type of importance)

'mating due to be re-evaluated' is the wordplay.
'mating due' anagrams to 'MAGNITUDE'.
But, I'm not sure how the anagram is indicated.

'of' acts as a link.

Can you help me to learn more?

(Other definitions for magnitude that I've seen before include "Eating mud (anag.)" , "(A star's) brightness" , "Dimensions" , "Largeness or size" , "Relative importance" .)

I've seen this clue in The Independent.

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