The most common and most likely answer to this crossword clue is the 6 letter word GLIDED
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D
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Buddy explains!

The straight part
We know that "moved smoothly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case moved smoothly = glided.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 GLIDED mean?
GLIDED verb- To move softly, smoothly, or effortlessly.
- To fly unpowered, as of an aircraft. Also relates to gliding birds and flying fish.
- To cause to glide.
- To pass with a glide, as the voice.