Family has different rate for protein (7)
I believe the answer is:
keratin
'for protein' is the definition.
(keratin is a kind of protein)
'family has different rate' is the wordplay.
'family' becomes 'kin' (synonyms).
'has' indicates putting letters inside.
'different' is an anagram indicator.
'rate' anagrammed gives 'erat'.
'kin' placed around 'erat' is 'KERATIN'.
(Other definitions for keratin that I've seen before include "Material in hooves and nails" , "Hair, nails substance" , "Protein found in hair and nails" , "Rate ink (anag) - fibrous protein" , "Constituent of hair/nails" .)