The most common and most likely answer to this crossword clue is the 5 letter word REDOX
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The straight part
We know that "chemical reaction" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case chemical reaction = redox.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 REDOX mean?
REDOX noun- A reaction in which an oxidation and a reduction occur simultaneously; a reaction in which electrons are transferred.
- To subject to a redox reaction.
- Of or relating to any reaction in which oxidation and reduction occur simultaneously.