Dull, taking top off, becoming flashy (4)
I believe the answer is:
loud
'flashy' is the definition.
(synonyms)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for loud that I've seen before include "Raucous" , "strongly audible" , "Blaring" , "Noisily showy?" , "Noisy or garish" .)