The most common and most likely answer to this crossword clue is the 4 plus 6 letter word OPEN SECRET
I've seen this crossword clue in The Guardian
O
P
E
N
S
E
C
R
E
T
More clues leading to the result OPEN SECRET
- Old submarine docks at eastern island — almost everyone knows it
- Lots of people know that present CEO has become corrupt
- Duck, swans, tercel mostly flap about – everyone knows it
- Unpublicised fact about golf championship that everyone knows?
- Everyone knows that duck and swans on east end of island disappeared
- Respect one broadcasting what’s already well known
- So pretence is ridiculous, if this?
- Generally known matter, so pretence silly
- Strange presence in part of Bible that everybody knows
- Choose to conceal vandalised censer with key that is well known – don’t mention it
Buddy explains!
The straight part
We know that "that everyone knows?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that everyone knows? = open secret.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.