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' is a reversal indicator.
'eenk' is hidden in the letters of 'seen kicking'.
'eenk' back-to-front 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" .)