The most common and most likely answer to this crossword clue is the 17 letter word NEEDLE-AND-THREAD
N
E
E
D
L
E
-
A
N
D
-
T
H
R
E
A
D
Buddy explains!
The straight part
We know that "couple found by a/sewer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case couple found by a/sewer = needle-and-thread.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.