Part of leg seen kicking backwards, but only part (4)
I believe the answer is:
knee
'part of leg' is the definition.
(I know that knee is part of leg)
'seen kicking backwards but only part' is the wordplay.
'but only part' indicates a hidden word.
'backwards' shows that the letters should be reversed in order.
'eenk' is hidden in the letters of 'seen kicking'.
'eenk' backwards is 'KNEE'.
(Other definitions for knee that I've seen before include "Leg?joint" , "Hinge between the thigh and the shin" , "middle part of leg" , "Joint of the body" , "for going on begging" .)