Upset with what one needs to know when buying headgear (7)
I believe the answer is:
capsize
This clue is a double definition.
'upset' is the first definition.
(I've seen this before)
'what one needs to know when buying headgear' is the second definition.
(a cap size, size for a cap)
'with' acts as a link.
(Other definitions for capsize that I've seen before include "Overturn (a boat)" , "Of boat, turn turtle" , "Upset" , "Turn so that keel faces upwards" .)