The most common and most likely answer to this crossword clue is the 4 letter word GUNK
I've seen this crossword clue in Independent
G
U
N
K
Buddy explains!

The straight part
We know that "Perhaps discharge" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case perhaps discharge = gunk.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 GUNK mean?
GUNK noun- Dirt or grime; any vague or unknown substance
- A subculture of 21st century American males, combining elements of modern gothic culture with punk rock.
- A member of the gunk subculture.
- To soil or make dirty