The most common and most likely answer to this crossword clue is the 5 letter word SUNUP
I've seen this crossword clue in The Sunday Telegraph
S
U
N
U
P
More clues leading to the result SUNUP
- Dawn‘s body found in drink
- What happens soon after Aurora raises matter involving a Parisian
- The lad is heard stirring at dawn!
- Dawn gets boy out of bed, we hear
- When we first see the light daily, no longer sleeping
- Rag at university that sees the night out
- When star makes first appearance in part of trip, unusually going North
- Oriental enlightenment?
- Dawn French for one is in short superlative
- Joke with us, flipping first light
Buddy explains!

The straight part
We know that "When we first see the light" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when we first see the light = sunup.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 SUNUP mean?
SUNUP noun- The time of day when the sun appears above the eastern horizon.
- The change in color of the sky at sunup.