The most common and most likely answer to this crossword clue is the 7 letter word ACUTEST
A
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Buddy explains!
The straight part
We know that "most serious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most serious = acutest.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 ACUTEST mean?
ACUTEST adjective- Brief, quick, short.
- High or shrill.
- Intense, sensitive, sharp.
- Urgent.
- With the sides meeting directly to form an acute angle (at an apex or base).
- Of an angle: less than 90 degrees.
- Of a triangle: having all three interior angles measuring less than 90 degrees.
- Of an accent or tone: generally higher than others.
- Of an abnormal condition of recent or sudden onset, in contrast to delayed onset; this sense does not imply severity, unlike the common usage.
- Of a short-lived condition, in contrast to a chronic condition; this sense also does not imply severity.
- After a letter of the alphabet: having an acute accent.