Dark part of a Mexican hat (6)
I believe the answer is:
sombre
'dark' is the definition.
(I've seen this before)
'part of a mexican hat' is the wordplay.
I cannot really understand how this works, but
'part' could be 'so' (so is a kind of part) and 'so' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sombre that I've seen before include "Lugubrious" , "Dark and gloomy of manner" , "Grave or gloomy" , "Gloomy and dismal" , "Dark and dismal" .)