The most common and most likely answer to this crossword clue is the 9 letter word DAYLIGHTS
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Buddy explains!
The straight part
We know that "sanity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sanity = daylights.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 DAYLIGHTS mean?
DAYLIGHTS noun- The light from the Sun, as opposed to that from any other source.
- A light source that simulates daylight.
- (photometry) The intensity distribution of light over the visible spectrum generated by the Sun under various conditions or by other light sources intended to simulate natural daylight.
- The period of time between sunrise and sunset.
- Daybreak.
- Exposure to public scrutiny.
- A clear, open space.
- (machinery) The space between platens on a press or similar machinery.
- Emotional or psychological distance between people, or disagreement.