The most common and most likely answer to this crossword clue is the 8 letter word TITRATES
I've seen this crossword clue in The Guardian
T
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More clues leading to the result TITRATES
Buddy explains!
The straight part
We know that "monitors and adjusts a chemical" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case monitors and adjusts a chemical = titrates.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 TITRATES mean?
TITRATES verb- To ascertain the amount of a constituent in a solution (or other mixture) by measuring the volume of a known concentration (the "standard solution") needed to complete a reaction.
- To adjust the amount of a drug consumed until the desired effects are achieved.