The most common and most likely answer to this crossword clue is the 5 letter word DILLY
D
I
L
L
Y
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The straight part
We know that "vacillate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vacillate = dilly.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 DILLY mean?
DILLY adjective- Redolent of dill (the herb).
- Someone or something that is remarkable or unusual.
- A dilly bag.
- A kind of stagecoach.