The most common and most likely answer to this crossword clue is the 8 letter word PEPTIDES
I've seen this crossword clue in The Times
P
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P
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S
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The straight part
We know that "chemical substances" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case chemical substances = peptides.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 PEPTIDES mean?
PEPTIDES noun- Any of a class of organic compounds consisting of various numbers of amino acids in which the amine of one is reacted with the carboxylic acid of the next to form an amide bond.
- The peptide bond itself.