Protein in excess ends in excretion (6)
I believe the answer is:
gluten
'protein' is the definition.
(gluten is a kind of protein)
'in excess ends in excretion' is the wordplay.
'in' means one lot of letters goes inside another.
'excess' becomes 'glut' (I've seen this before).
'ends' indicates one should take the final letters.
'excretion' becomes 'e' (this could be a standard abbreviation of which I'm unaware).
The last letter of 'in' is 'n'.
'glut'+'n'='glutn'
'glutn' enclosing 'e' is 'GLUTEN'.
Can you help me to learn more?
(Other definitions for gluten that I've seen before include "Protein present in wheat" , "Protein in wheat grains" , "Protein found in cereals" , "Substance in cereal grain" , "Part of wheat" .)