The most common and most likely answer to this crossword clue is the 8 letter word FREELOAD
I've seen this crossword clue in The Guardian
F
R
E
E
L
O
A
D
Buddy explains!

The straight part
We know that "Bum" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bum = freeload.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 FREELOAD mean?
FREELOAD verb- To live off the generosity or hospitality of others