The most common and most likely answer to this crossword clue is the 4 plus 2 plus 1 plus 4 plus 4 letter word ONCE IN A BLUE MOON
I've seen this crossword clue in Independent
O
N
C
E
I
N
A
B
L
U
E
M
O
O
N
More clues leading to the result ONCE IN A BLUE MOON
- Cryptic clue in one newspaper’s leader: ‘a boom with extremely low frequency‘
- Seldom working, Church girl unhappy with short time available
- Love, with noble union, came sadly very rarely
- Hardly ever come with a lone bunion needing treatment
- Under the influence of drugs, I take solvent, remove top and expose myself occasionally
- A Tory gets in no income irregularly, working very occasionally
- Rarely alone? No, I’m one cub that’s been abandoned
- British involved in stirring up EU malice? No, no, no . . . . hardly ever
- When at work, an unhappy doctor gets about very rarely
- Crumble onion cube on meal on rare occasions
Buddy explains!
The straight part
We know that "with extremely low frequency" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case with extremely low frequency = once in a blue moon.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.