Bone seeing top part twisting back (5)
I believe the answer is:
tibia
'bone' is the definition.
(I know that tibia is a type of long bone)
'top part twisting back' is the wordplay.
'top' becomes 'ai' (I've seen this in other clues).
'part' becomes 'bit part' (both mean a piece of something).
'twisting back' says the letters should be written backwards.
'ai'+'bit'='aibit'
'aibit' reversed gives 'TIBIA'.
'seeing' is the link.
(Other definitions for tibia that I've seen before include "Bone in osso bucco" , "The large shinbone" , "ancient pipe" , "Bone of the lower leg" , "Part of leg of" .)