The most common and most likely answer to this crossword clue is the 12 letter word HABERDASHERY
I've seen this crossword clue in Financial Times
H
A
B
E
R
D
A
S
H
E
R
Y
Buddy explains!

The straight part
We know that "Stuff fit for the sewer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stuff fit for the sewer = haberdashery.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 HABERDASHERY mean?
HABERDASHERY noun- Ribbons, buttons, thread, needles and similar sewing goods sold in a haberdasher's shop.
- A shop selling such goods.
- A shop selling clothing and accessories for men, including hats.