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Matters which have remained unsettled in Red issue (8)

Ross

I believe the answer is:

residues

I'm a little stuck... Click here to teach me more about this clue!

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

'have remained unsettled in red issue' is the wordplay.
'red issue' can be anagrammed to 'RESIDUES'.
But, I am not clear how the anagram is indicated.

'which' is the link.

Can you help me to learn more?

(Other definitions for residues that I've seen before include "Remains after something has been removed" , "Leftovers, dregs" , "More than one thing left behind" , "Remainders" , "They remain after things has been removed" .)

I've seen this clue in The Times and The Sunday Times.
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