Brown logs American left to rot (5)

I believe the answer is:

umber

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'brown' is the definition.
(I know that umber is a type of brown)

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

(Other definitions for umber that I've seen before include "Dark brown coloring matter" , "Yellowish-brown pigment" , "Yellow-brown pigment" , "Brown earthy substance used as pigment" , "Natural colouring matter, dark brown" .)

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

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