Green recycling by everyone causes bad reaction (8)
I believe the answer is:
allergen
'bad reaction' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'green recycling by everyone' is the wordplay.
'recycling' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'everyone' becomes 'all' (all people).
'green' with letters rearranged gives 'ergen'.
'ergen' after 'all' is 'ALLERGEN'.
'causes' is the link.
Can you help me to learn more?
(Other definitions for allergen that I've seen before include "Substance to which body reacts" , "One provokes a reaction" , "that may produce adverse reaction" , "Wheat, say" , "It produces a damaging response" .)