Dark hat last couple is missing (6)
I believe the answer is:
sombre
'dark' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for sombre that I've seen before include "Grave and serious" , "Funereal" , "Dim or dismal" , "Lugubrious" , "Dark and gloomy of manner" .)