The most common and most likely answer to this crossword clue is the 6 letter word VAILED
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Buddy explains!
The straight part
We know that "Once in disguise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case once in disguise = vailed.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 VAILED mean?
VAILED verb- To pay homage, bow, submit, defer (to someone or something); to yield, give way (to something).
- To remove as a sign of deference, as a hat.
- To lower, let fall; to allow or cause to sink.
- (vexillology) (current, operational) To lower or "dip" a carried flag or banner in a salute by a forward reducing of the angle of the pike/flagstaff with respect to the ground; in extreme instances, as when saluting a monarch, both the banner and the finial of the pike are allowed to rest upon the ground.
- To dress in, or decorate with, a veil.
- To conceal as with a veil.