The most common and most likely answer to this crossword clue is the 6 plus 3 letter word LIGHTS OUT
I've seen this crossword clue in The Sunday Telegraph
L
I
G
H
T
S
O
U
T
More clues leading to the result LIGHTS OUT
Buddy explains!

The straight part
We know that "end of the day" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case end of the day = lights out.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 LIGHTS OUT mean?
LIGHTS OUT noun- The time at night at which artificial lights are to be turned off, a curfew or bedtime.
- The prescribed bedtime for persons living in a boarding school or staying in a hospital.