The most common and most likely answer to this crossword clue is the 6 letter word WAGGLE
I've seen this crossword clue in Independent
W
A
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Buddy explains!
The straight part
We know that "Shake rapidly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shake rapidly = waggle.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 WAGGLE mean?
WAGGLE noun- A wobbling motion.
- The preliminary swinging of the club head back and forth over the ball in the line of the proposed stroke.
- To move (something) with short, quick motions; to wobble.
- To reel, sway, or move from side to side; to move with a wagging motion; to waddle.