The most common and most likely answer to this crossword clue is the 8 letter word FLATNESS
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!
The straight part
We know that "Boring quality" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case boring quality = flatness.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 FLATNESS mean?
FLATNESS noun- The state of being flat
- The state of being two-dimensional; planar : planarity
- The state of being bland : dullness
- Mattness, the quality of a painted surface which scatters or absorbs the light falling on it, so as to be substantially free from gloss or sheen
- Lack of bouquet and freshness of a wine, through too much aeration or infection with film yeasts
- Slim-tailedness or platykurticity