The most common and most likely answer to this crossword clue is the 5 letter word WANED
I've seen this crossword clue in The Daily Telegraph
W
A
N
E
D
More clues leading to the result WANED
Buddy explains!

The straight part
We know that "declined" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case declined = waned.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 WANED mean?
WANED verb- To progressively lose its splendor, value, ardor, power, intensity etc.; to decline.
- Said of light that dims or diminishes in strength.
- Said of the Moon as it passes through the phases of its monthly cycle where its surface is less and less visible.
- Said of a time period that comes to an end.
- To decrease physically in size, amount, numbers or surface.
- To cause to decrease.
- Having wanes, i.e. rounded corners caused by lack of wood, often showing bark.