Gloomy Republican with old wide-brimmed hat (8)
I believe the answer is:
sombrero
'wide-brimmed hat' is the definition.
(I've seen this before)
'gloomy republican with old' is the wordplay.
'gloomy' becomes 'sombre' (I've seen this before**).
'republican' becomes 'r' (abbreviation).
'with' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sombre'+'r'+'o'='SOMBRERO'
(Other definitions for sombrero that I've seen before include "Wide-brimmed 23" , "headgear from Mexico" , "Broad-brimmed Mexican hat" , "Mexican's hat" , "CLOTHING" .)