Doctor lancing boil's ultimately rather old hat (8)
I believe the answer is:
sombrero
'rather old hat' is the definition.
'sombrero' can be an answer for 'hat' (sombrero is a kind of hat). I'm not certain of the 'rather old' bit.
'doctor lancing boil's ultimately' is the wordplay.
'doctor' becomes 'MB' (Bachelor of Medicine).
'lancing' is an insertion indicator (some letters cut or lance into others).
'boil' becomes 'sore' (boil can mean a sore or inflammation).
'ultimately' becomes 'ro' (I am not sure about this - if you are sure you should believe this answer much more).
'mb' going into 'sore' is 'sombre'.
'sombre'+'ro'='SOMBRERO'
Can you help me to learn more?
(Other definitions for sombrero that I've seen before include "a Mexican's overheads" , "Large hat from Mexico" , "Wide-brimmed Mexican hat" , "Mexican's hat" , "headgear from Mexico" .)