The most common and most likely answer to this crossword clue is the 8 letter word THIOUREA
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The straight part
We know that "thyroid inhibitor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thyroid inhibitor = thiourea.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 THIOUREA mean?
THIOUREA noun- Any of a class of compounds based on NH2-CS-NH2, formally derived from urea by replacing the oxygen atom with sulfur, used in photography as a fixing agent, in inorganic synthesis, and in medicine as an antithyroid drug.