Dull headgear ultimately — old, it may be felt (8)
I believe the answer is:
sombrero
'it may be felt' is the definition.
(I've seen this before)
'dull headgear ultimately old' is the wordplay.
'dull' becomes 'sombre' ().
'ultimately' suggests the final letters.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
The last letter of 'headgear' is 'r'.
'sombre'+'r'+'o'='SOMBRERO'
(Other definitions for sombrero that I've seen before include "Hat for South American" , "Wide-brimmed hat of Mexico" , "Large Mexican sun-hat" , "It may be felt" , "Large hat from Mexico" .)