The most common and most likely answer to this crossword clue is the 7 letter word SUNRISE
I've seen this crossword clue in The Guardian
S
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N
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More clues leading to the result SUNRISE
- Dawn and Susan eating near island
- Dawn insures convertible
- English teacher and students turning up so early in the day?
- Nurses treated current stabbing early in the day
- Dawn French, one in funny series she’s finally left
- Dawn run unfortunately is limited by a lot of sex
- English teacher and students getting up when day breaks
- Rebuild ruins in the Home Counties for star’s first appearance of the day
- Light moment?
- Dawn’s boy heard at the beginning
Buddy explains!

The straight part
We know that "when day breaks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when day breaks = sunrise.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 SUNRISE mean?
SUNRISE noun- The time of day when the sun appears above the eastern horizon.
- The change in color of the sky at dawn.
- Any great awakening.