The most common and most likely answer to this crossword clue is the 5 plus 8 letter word BOWEL MOVEMENT
I've seen this crossword clue in The Guardian
B
O
W
E
L
M
O
V
E
M
E
N
T
Buddy explains!
The straight part
We know that "Dump" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dump = bowel movement.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.