Fingernail's fibrous protein (7)

I believe the answer is:

keratin

(Other definitions for keratin that I've seen before include "Protein found in hair and nails" , "Hair, nails substance" , "Constituent of hair/nails" , "Rate ink (anag) - fibrous protein" , "Material in hooves and nails" .)

I've seen this clue in the Sydney Morning Herald.

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