The most common and most likely answer to this crossword clue is the 8 letter word HYPNOTIC
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The straight part
We know that "Mesmerising" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mesmerising = hypnotic.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 HYPNOTIC mean?
HYPNOTIC noun- A person who is, or can be, hypnotized.
- A soporific substance.
- Of, or relating to hypnosis or hypnotism.
- (pharmaceutical effect) Inducing sleep; soporific.