Colour of excellent street guide (5)

I believe the answer is:

topaz

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'colour' is the definition.
(topaz is a kind of colour)

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

(Other definitions for topaz that I've seen before include "Type of gemstone" , "South American hummingbird" , "White or colourless gemstone" , "Yellow quartz used for gemstones" , "Yellow or pale blue gem" .)

I've seen this clue in The Telegraph.

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