The most common and most likely answer to this crossword clue is the 12 letter word DEGENERATION
I've seen this crossword clue in The Times
D
E
G
E
N
E
R
A
T
I
O
N
More clues leading to the result DEGENERATION
Buddy explains!
The straight part
We know that "Decline" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decline = degeneration.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 DEGENERATION mean?
DEGENERATION noun- The process or state of growing worse, or the state of having become worse.
- That condition of a tissue or an organ in which its vitality has become either diminished or perverted; a substitution of a lower for a higher form of structure.
- Gradual deterioration, from natural causes, of any class of animals or plants or any particular organ or organs; hereditary degradation of type.
- A thing that has degenerated.