The most common and most likely answer to this crossword clue is the 10 letter word NIGHT-TIME
I've seen this crossword clue in Independent
N
I
G
H
T
-
T
I
M
E
More clues leading to the result NIGHT-TIME
Buddy explains!

The straight part
We know that "darker hours" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case darker hours = night-time.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 NIGHT-TIME mean?
NIGHT-TIME noun- The hours of darkness between sunset and sunrise; the night.