The most common and most likely answer to this crossword clue is the 7 letter word DULLEST
I've seen this crossword clue in The Guardian
D
U
L
L
E
S
T
Buddy explains!

The straight part
We know that "Least exciting" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case least exciting = dullest.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 DULLEST mean?
DULLEST adjective- 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.