Formosa must assumingly partly counter Mandarin (7)

I believe the answer is:

satsuma

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'mandarin' is the definition.
(I know that satsuma is a type of mandarin)

I can't explain the rest of the clue.

(Other definitions for satsuma that I've seen before include "Type of mandarin orange" , "that is easily uncovered" , "that's segmented" , "Citrus fruit; Jap. pottery" , "Kind of tangerine" .)

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

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