Tastelessly loud and insolent supporters, extremely surly (6)
I believe the answer is:
brassy
'tastelessly loud and insolent' is the definition.
'supporters extremely surly' is the wordplay.
'supporters' becomes 'bras' (I've seen this in other clues).
'extremely' means to remove the middle letters.
'surly' with its centre removed is 'sy'.
'bras'+'sy'='BRASSY'
(Other definitions for brassy that I've seen before include "Sounding like cornet maybe" , "Bold and vulgar like yellow metal" , "Brazen-faced - harsh" , "Cocksure" , "Tastelessly showy, loud" .)