The most common and most likely answer to this crossword clue is the 4 letter word WHOP
W
H
O
P
Buddy explains!
The straight part
We know that "Sound of a bump" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sound of a bump = whop.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 WHOP mean?
WHOP noun- A blow or strike.
- To throw or move (something) quickly, usually with an impact.
- To administer corporal punishment