Dull or retro headgear (8)

I believe the answer is:

sombrero

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'headgear' is the definition.
(I know that sombrero is a type of hat)

'dull or retro' is the wordplay.
'dull' becomes 'sombre' (I can't justify this - if you can you should give a lot more credence to this answer).
'retro' says the letters should be written backwards (retro is Latin for 'backwards').
'or' back-to-front is 'ro'.
'sombre'+'ro'='SOMBRERO'

Can you help me to learn more?

(Other definitions for sombrero that I've seen before include "It may be felt" , "a Mexican's overheads" , "Covering up" , "Broad-brimmed Mexican hat" , "One sported in sun" .)

I've seen this clue in The Telegraph.

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