Walker has left out warning signal (5)

I believe the answer is:

amber

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

'warning signal' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for amber that I've seen before include "Honey-yellow colour" , "Orange traffic light" , "Yellow-brown colour as of bream" , "Brownish yellow" , "Yellow-orange" .)

I've seen this clue in The Times.

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