Dark dresser, ultimately old hat (8)
I believe the answer is:
sombrero
'hat' is the definition.
(sombrero is a kind of hat)
'dark dresser ultimately old' is the wordplay.
'dark' becomes 'sombre' (I've seen this before).
'ultimately' indicates one should take the final letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
The last letter of 'dresser' is 'r'.
'sombre'+'r'+'o'='SOMBRERO'
(Other definitions for sombrero that I've seen before include "Mexican's hat" , "It may be felt" , "Wide-brimmed 23" , "Straw hat with very wide brim from Mexico" , "Wide-brimmed hat of Mexico" .)