Hair protein (7)
I believe the answer is:
keratin
'hair protein' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for keratin that I've seen before include "Material in hooves and nails" , "Hair, nails substance" , "Constituent of hair/nails" , "Protein found in hair and nails" , "Rate ink (anag) - fibrous protein" .)