Not up to it — except when wearing headgear? (9)
I believe the answer is:
incapable
'not up to it' is the definition.
(not up to something means being unable to do it)
'wearing headgear?' is the wordplay.
'wearing' becomes 'in' (as in being 'in' certain clothes).
'headgear?' becomes 'capable' (I can't explain this - if you can you should give a lot more credence to this answer).
'in'+'capable'='INCAPABLE'
'except when' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for incapable that I've seen before include "Extremely drunk" , "Possibly hammered" , "Useless" , "Unable, incompetent" , "lacking ability" .)