The most common and most likely answer to this crossword clue is the 9 letter word PENUMBRAL
I've seen this crossword clue in The Guardian
P
E
N
U
M
B
R
A
L
Buddy explains!
The straight part
We know that "Semi-dim" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case semi-dim = penumbral.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.