The most common and most likely answer to this crossword clue is the 8 letter word ALLERGEN
I've seen this crossword clue in The Times
A
L
L
E
R
G
E
N
More clues leading to the result ALLERGEN
- Generally incomplete, with dope and everything switched, it produces nasty reaction
- Everyone turned green, finding a harmful substance
- Whole green bananas can cause a bad reaction
- . Claim new clothes start to reduce cause of bad reaction
- Film director penning bit of work — it provokes a reaction
- Everything green made troublesome pollen, say
- It’ll provoke a reaction from smaller gentlemen
- Generally being short-tempered is a source of adverse reaction
- Everything’s green, may be it will trigger a reaction
- Woody surrounding for work unit that produces a reaction?
Buddy explains!
The straight part
We know that "it produces nasty reaction" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it produces nasty reaction = allergen.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ALLERGEN mean?
ALLERGEN noun- A substance which causes an allergic reaction.