Kneecap of one girl seen by another (7)
I believe the answer is:
patella
'kneecap' is the definition.
(I know that kneecap can be written as patella)
'one girl seen by another' is the wordplay.
'one girl' becomes 'Ella' (girl's name).
'seen by' is a charade indicator (letters next to each other).
'another' becomes 'pat' (I am not sure about this - if you are sure you should believe this answer much more).
'ella' put after 'pat' is 'PATELLA'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for patella that I've seen before include "Kneecap (anatomy)" , "A pallet (anag.)" , "The formal kneecap" , "Knee bone" , "bit of body usually under trousers!" .)