The most common and most likely answer to this crossword clue is the 4 letter word DULL
I've seen this crossword clue in The Guardian
D
U
L
L
More clues leading to the result DULL
Buddy explains!
The straight part
We know that "Definitely uninteresting, lacking lustre on top" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case definitely uninteresting, lacking lustre on top = dull.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 DULL mean?
DULL verb- To render dull; to remove or blunt an edge or something that was sharp.
- To soften, moderate or blunt; to make dull, stupid, or sluggish; to stupefy.
- To lose a sharp edge; to become dull.
- To render dim or obscure; to sully; to tarnish.
- Lacking the ability to cut easily; not sharp.
- Boring; not exciting or interesting.
- Not shiny; having a matte finish or no particular luster or brightness.
- Not bright or intelligent; stupid; having slow understanding.
- Sluggish, listless.
- Cloudy, overcast.
- Insensible; unfeeling.
- Heavy; lifeless; inert.
- (of pain etc) Not intense; felt indistinctly or only slightly.
- (of a noise or sound) Not clear, muffled.