Top class piece of information about bone (5)
I believe the answer is:
tibia
'bone' is the definition.
(I know that tibia is a bone)
'top class piece of information about' is the wordplay.
'top class' becomes 'AI' (resembles Ab, old-fashioned term meaning excellent).
'piece of information' becomes 'bit' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' is a reversal indicator.
'ai'+'bit'='aibit'
'aibit' written backwards gives 'TIBIA'.
Can you help me to learn more?
(Other definitions for tibia that I've seen before include "Bone of the lower leg" , "ancient pipe" , "The formal shinbone" , "The large shinbone" , "Part of leg of" .)