Most of broad hat is dark (6)

I believe the answer is:

sombre

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

I can't explain the rest of the clue.

(Other definitions for sombre that I've seen before include "Joyless" , "Grave or gloomy" , "Gloomy and dismal" , "Dark and gloomy of manner" , "Melancholy" .)

I've seen this clue in The Guardian.

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