The most common and most likely answer to this crossword clue is the 5 letter word PALLS
I've seen this crossword clue in The Daily Telegraph
P
A
L
L
S
Buddy explains!

The straight part
We know that "becomes less interesting" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case becomes less interesting = palls.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 PALLS mean?
PALLS noun- Senses relating to cloth.
- Senses relating to clothing.
- To cloak or cover with, or as if with, a pall.
- To make vapid or insipid; to make lifeless or spiritless; to dull, to weaken.
- To become dull, insipid, tasteless, or vapid; to lose life, spirit, strength, or taste.
- A feeling of nausea caused by disgust or overindulgence.