The most common and most likely answer to this crossword clue is the 4 plus 3 plus 3 letter word STOP THE ROT
I've seen this crossword clue in The Guardian
S
T
O
P
T
H
E
R
O
T
Buddy explains!
The straight part
We know that "end decline /(4,3,3)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case end decline /(4,3,3) = stop the rot.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.