The most common and most likely answer to this crossword clue is the 10 letter word HALF-LIGHT
I've seen this crossword clue in Independent
H
A
L
F
-
L
I
G
H
T
More clues leading to the result HALF-LIGHT
Buddy explains!
The straight part
We know that "semi-darkness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case semi-darkness = half-light.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 HALF-LIGHT mean?
HALF-LIGHT noun- The soft, greyish light characteristic of dawn or dusk