The most common and most likely answer to this crossword clue is the 6 letter word STOOLS
I've seen this crossword clue in The Guardian
S
T
O
O
L
S
Buddy explains!
The straight part
We know that "Waste product" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case waste product = stools.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 STOOLS mean?
STOOLS noun- A seat, especially for one person and without armrests.
- A close-stool; a seat used for urination and defecation: a chamber pot, commode, outhouse seat, or toilet.
- A plant that has been cut down until its main stem is close to the ground, resembling a stool, to promote new growth.
- Feces, excrement.
- A production of feces or excrement, an act of defecation, stooling.
- A decoy; a portable piece of wood to which a pigeon is fastened to lure wild birds.
- A small channel on the side of a vessel, for the deadeyes of the backstays.
- Material, such as oyster shells, spread on the sea bottom for oyster spat to adhere to.
- A plant from which layers are propagated by bending its branches into the soil.