The most common and most likely answer to this crossword clue is the 4 plus 4 letter word COCK CROW
I've seen this crossword clue in Financial Times
C
O
C
K
C
R
O
W
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The straight part
We know that "Dawn" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dawn = cock crow.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 COCK CROW mean?
COCK CROW noun- The time of day at which the first crow of a cockerel is heard; dawn or daybreak; first light