The most common and most likely answer to this crossword clue is the 11 letter word CONTRACTION
I've seen this crossword clue in The Guardian
C
O
N
T
R
A
C
T
I
O
N
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 = contraction.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 CONTRACTION mean?
CONTRACTION noun- A reversible reduction in size.
- A period of economic decline or negative growth.
- A shortening of a muscle when it is used.
- A strong and often painful shortening of the uterine muscles prior to or during childbirth.
- A process whereby one or more sounds of a free morpheme (a word) are lost or reduced, such that it becomes a bound morpheme (a clitic) that attaches phonologically to an adjacent word.
- (English orthography) A word with omitted letters replaced by an apostrophe, usually resulting from the above process.
- A shorthand symbol indicating an omission for the purpose of brevity.
- The process of contracting a disease.
- Syncope, the loss of sounds from within a word.
- The acquisition of something, generally negative.
- A distinct stage of wound healing, wherein the wound edges are gradually pulled together.