Too unfashionable to go to one's head (3,3)
I believe the answer is:
old hat
'too unfashionable' is the definition.
'old hat' can be an answer for 'unfashionable' (thesaurus). I am not sure about the 'too' bit.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for old hat that I've seen before include "Stale - out of date" , "Dated" , "could be past its sell-by date?" , "Out of date like ancient headgear" , "Past its prime, like ancient headgear" .)