The most common and most likely answer to this crossword clue is the 8 letter word DULLNESS
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The straight part
We know that "lack of interest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lack of interest = dullness.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 DULLNESS mean?
DULLNESS noun- The quality of being slow of understanding things; stupidity.
- The quality of being uninteresting; boring or irksome.
- Lack of interest or excitement.
- The lack of visual brilliance; want of sheen.
- (of an edge) bluntness.
- The quality of not perceiving or kenning things distinctly.
- Drowsiness.