Medic wears more painful old headgear (8)
I believe the answer is:
sombrero
'headgear' is the definition.
(sombrero is a kind of headgear)
'medic wears more painful old' is the wordplay.
'medic' becomes 'mb' (Bachelor of Medicine).
'wears' is an insertion indicator.
'more painful' becomes 'sorer' ('sore' can be similar in meaning to 'painful').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'mb' put within 'sorer' is 'sombrer'.
'sombrer'+'o'='SOMBRERO'
(Other definitions for sombrero that I've seen before include "Covering up" , "headgear from Mexico" , "Wide-brimmed hat of Mexico" , "CLOTHING" , "One sported in sun" .)