The most common and most likely answer to this crossword clue is the 8 letter word REAGENTS
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More clues leading to the result REAGENTS
Buddy explains!
The straight part
We know that "have reactionary tendencies" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case have reactionary tendencies = reagents.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 REAGENTS mean?
REAGENTS noun- A compound or mixture of compounds used to treat or test materials, samples, other compounds or reactants in a laboratory or sometimes an industrial setting.