The most common and most likely answer to this crossword clue is the 6 letter word SLUDGE
I've seen this crossword clue in The Daily Telegraph
S
L
U
D
G
E
Buddy explains!

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 SLUDGE mean?
SLUDGE noun- Solids separated from suspension in a liquid.
- A residual semi-solid material left from industrial, water treatment, or wastewater treatment processes.
- A sediment of accumulated minerals in a steam boiler.
- A mass of small pieces of ice on the surface of a body of water.
- Sludge metal
- To slump or slouch.
- To slop or drip slowly.