Not, you say, the green collar (4)

I believe the answer is:

ruff

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'collar' is the definition.
(I know that ruff is a type of collar)

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

(Other definitions for ruff that I've seen before include "Jacobean neckwear" , "Elizabethan or Jacobean neck adornment" , "High tight collar" , "Type of collar" , "Starched frill; fish" .)

I've seen this clue in the Evening Standard.

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