The most common and most likely answer to this crossword clue is the 6 letter word DROOPS
I've seen this crossword clue in The Daily Telegraph
D
R
O
O
P
S
More clues leading to the result DROOPS
Buddy explains!

The straight part
We know that "Declines" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case declines = droops.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 DROOPS mean?
DROOPS verb- To hang downward; to sag.
- To slowly become limp; to bend gradually.
- To lose all energy, enthusiasm or happiness; to flag.
- To allow to droop or sink.
- To proceed downward, or toward a close; to decline.