The head? All wrong. It's part of the knee (7)
I believe the answer is:
patella
'part of the knee' is the definition.
('patella' is part of 'knee')
'the head? all wrong' is the wordplay.
'the head?' becomes 'pate' (humorous term for the head).
'wrong' is an anagram indicator.
'all' anagrammed gives 'lla'.
'pate'+'lla'='PATELLA'
'it's' is the link.
(Other definitions for patella that I've seen before include "Joint cover" , "Kneecap (anatomy)" , "showing a bit of leg" , "positioned on each knee" , "Knee bone" .)