Allergen: excessive supply ends in some reaction (6)
I believe the answer is:
gluten
'allergen' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'excessive supply ends in some reaction' is the wordplay.
'excessive supply' becomes 'glut' (I've seen this in another clue).
'ends in' means to remove the middle letters (only the letters at the ends of the words).
'some reaction' becomes 'emotion' (I've seen this before).
'emotion' with its middle taken out is 'en'.
'glut'+'en'='GLUTEN'
Can you help me to learn more?
(Other definitions for gluten that I've seen before include "Protein substance from cereal grains that gives elasticity to dough" , "Substance in cereals" , "Protein present in wheat" , "Protein source" , "Protein in wheat grains" .)