Bone's top piece put back (5)
I believe the answer is:
tibia
'bone's' is the definition.
(tibia is a kind of bone)
'top piece put back' is the wordplay.
'top' becomes 'ai' (resembles A1, term for excellent).
'piece' becomes 'bit' (bit is a kind of piece).
'put back' says the letters should be written backwards.
'ai'+'bit'='aibit'
'aibit' back-to-front is 'TIBIA'.
(Other definitions for tibia that I've seen before include "Part of leg of" , "Shin bone" , "ancient pipe" , "The formal shinbone" , "The large shinbone" .)