The most common and most likely answer to this crossword clue is the 10 letter word NOSEDIVING
I've seen this crossword clue in The Daily Telegraph
N
O
S
E
D
I
V
I
N
G
Buddy explains!
The straight part
We know that "in rapid decline" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in rapid decline = nosediving.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 NOSEDIVING mean?
NOSEDIVING verb- (of aircraft) To dive down in a steep angle.
- To perform a rapid fall in price or value.