The most common and most likely answer to this crossword clue is the 5 letter word DAWNS
I've seen this crossword clue in Independent
D
A
W
N
S
More clues leading to the result DAWNS
Buddy explains!

The straight part
We know that "gets lighter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gets lighter = dawns.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 DAWNS mean?
DAWNS noun- The morning twilight period immediately before sunrise.
- The rising of the sun.
- The time when the sun rises.
- The earliest phase of something.
- To begin to brighten with daylight.
- To start to appear or be realized.
- To begin to give promise; to begin to appear or to expand.