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

The straight part
We know that "gone into a decline" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gone into a decline = ebbed.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 EBBED mean?
EBBED verb- To flow back or recede
- To fall away or decline
- To fish with stakes and nets that serve to prevent the fish from getting back into the sea with the ebb
- To cause to flow back.