Hair protein (7)
I believe the answer is:
keratin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">hair protein</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)