The most common and most likely answer to this crossword clue is the 6 letter word BLEARS
I've seen this crossword clue in The Guardian
B
L
E
A
R
S
More clues leading to the result BLEARS
Buddy explains!

The straight part
We know that "Makes dim" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case makes dim = blears.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 BLEARS mean?
BLEARS verb- To be blear; to have blear eyes; to look or gaze with blear eyes.
- (of the eyes or eyesight) To make blurred or dim.
- (of an image) To blur, make blurry.