The most common and most likely answer to this crossword clue is the 9 letter word DOWNWARDS
I've seen this crossword clue in The Guardian
D
O
W
N
W
A
R
D
S
More clues leading to the result DOWNWARDS
Buddy explains!

The straight part
We know that "declining" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case declining = downwards.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 DOWNWARDS mean?
DOWNWARDS adverb- Towards a lower place; towards what is below.
- Towards something which is lower in order, smaller, inferior, etc.