Hard cap taken from head, all upside down (7)
I believe the answer is:
patella
'hard cap' is the definition.
(knee cap)
'head all upside down' is the wordplay.
'head' becomes 'pate' (humorous term for the head).
'upside down' says the letters should be written backwards.
'all' backwards is 'lla'.
'pate'+'lla'='PATELLA'
'taken from' acts as a link.
(Other definitions for patella that I've seen before include "showing a bit of leg" , "Kneecap (anatomy)" , "bit of body usually under trousers!" , "The formal kneecap" , "Knee bone" .)