When climbing cracking section in a bone (5)
I believe the answer is:
tibia
'a bone' is the definition.
(tibia is a kind of bone)
'when climbing cracking section' is the wordplay.
'when climbing' shows that the letters should be reversed in order (in a down clue, letters go up).
'cracking' becomes 'ai' (resembles A1, informal term for excellent).
'section' becomes 'bit' (I've seen this in other clues).
'ai'+'bit'='aibit'
'aibit' in reverse letter order is 'TIBIA'.
'in' is the link.
(Other definitions for tibia that I've seen before include "Part of leg of" , "The formal shinbone" , "The large shinbone" , "ancient pipe" , "Bone in osso bucco" .)