Crossword Genius

___ in a blue moon (4)

Ross

I believe the answer is:

(Other definitions for once that I've seen before include "One-time musical film and song" , "In the old days" , "At a previous time" , "in bygone days" , "Formerly - not repeated" .)

I've seen this clue in the Universal, the USA Today, the Newsday, The Mirror and the LA Times.
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