The most common and most likely answer to this crossword clue is the 8 letter word THICKEST
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More clues leading to the result THICKEST
Buddy explains!

The straight part
We know that "Dullest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dullest = thickest.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 THICKEST mean?
THICKEST adjective- Relatively great in extent from one surface to the opposite in its smallest solid dimension.
- Measuring a certain number of units in this dimension.
- Heavy in build; thickset.
- Densely crowded or packed.
- Having a viscous consistency.
- Abounding in number.
- Impenetrable to sight.
- (Of an accent) Prominent, strong.
- Stupid.
- Friendly or intimate.
- Deep, intense, or profound.
- Troublesome; unreasonable
- (chiefly of women) Curvy and voluptuous, and especially having large hips.
- In a thick manner.
- Frequently or numerously.