Hat, gloomy in colour — the reverse of golden (8)
I believe the answer is:
sombrero
'hat' is the definition.
(sombrero is a kind of hat)
'gloomy in colour the reverse of golden' is the wordplay.
'gloomy in colour' becomes 'sombre' (I can't explain this - if you can you should give a lot more credence to this answer).
'the reverse of' shows that the letters should be reversed in order.
'golden' becomes 'or'.
'or' written backwards gives 'ro'.
'sombre'+'ro'='SOMBRERO'
Can you help me to learn more?
(Other definitions for sombrero that I've seen before include "Straw hat with very wide brim from Mexico" , "One sported in sun" , "Mexican's hat" , "It may be felt" , "Large Mexican sun-hat" .)