The most common and most likely answer to this crossword clue is the 9 letter word DEADLIEST
I've seen this crossword clue in The Times
D
E
A
D
L
I
E
S
T
More clues leading to the result DEADLIEST
Buddy explains!

The straight part
We know that "of least interest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of least interest = deadliest.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 DEADLIEST mean?
DEADLIEST adjective- Subject to death; mortal.
- Causing death; lethal.
- Aiming or willing to destroy; implacable; desperately hostile.
- Very accurate (of aiming with a bow, firearm, etc.).
- Very boring.
- Excellent, awesome, cool.