The most common and most likely answer to this crossword clue is the 7 letter word FATBERG
I've seen this crossword clue in The Guardian
F
A
T
B
E
R
G
Buddy explains!
The straight part
We know that "Mass of waste" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mass of waste = fatberg.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 FATBERG mean?
FATBERG noun- A large accumulation of fat and discarded toiletries which clogs sewers.