The most common and most likely answer to this crossword clue is the 4 plus 3 plus 7 letter word HUNT THE THIMBLE
I've seen this crossword clue in The Daily Telegraph
H
U
N
T
T
H
E
T
H
I
M
B
L
E
Buddy explains!
The straight part
We know that "Diversion/that could put the sewer temporarily out of action" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case diversion/that could put the sewer temporarily out of action = hunt the thimble.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.