Crossword Genius

The central body in the solar system scarlet-backed is separate (6)

Ross

I believe the answer is:

sunder

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'is separate' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

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(Other definitions for sunder that I've seen before include "Nursed so as to tear apart" , "Break in pieces" , "Separate the odd red sun" , "Red sun could part" , "Tear apart, sever" .)

I've seen this clue in the Sydney Morning Herald.
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