The most common and most likely answer to this crossword clue is the 6 letter word SLOWER
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More clues leading to the result SLOWER
Buddy explains!

The straight part
We know that "Dimmer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dimmer = slower.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 SLOWER mean?
SLOWER adjective- Taking a long time to move or go a short distance, or to perform an action; not quick in motion; proceeding at a low speed.
- Not happening in a short time; spread over a comparatively long time.
- Of reduced intellectual capacity; not quick to comprehend.
- Not hasty; not tending to hurry; acting with deliberation or caution.
- (of a clock or the like) Behind in time; indicating a time earlier than the true time.
- Lacking spirit; deficient in liveliness or briskness.
- (of a period of time) Not busy; lacking activity.
- Slowly.
- That which slows.