Mountain's substance: keratin (10)
I believe the answer is:
matterhorn
'mountain's' is the definition.
(I've seen this before)
'substance keratin' is the wordplay.
I cannot really see how this works, but
'substance' could be 'matter' (synonyms) and 'matter' is present in the answer.
The remaining letters 'horn' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for matterhorn that I've seen before include "Mountain on the Swiss-Italian border" , "A scale that may be difficult" , "one of a range" , "peak" , "Alpine challenge" .)