The most common and most likely answer to this crossword clue is the 5 letter word CADGE
I've seen this crossword clue in The Guardian
C
A
D
G
E
More clues leading to the result CADGE
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 = cadge.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 CADGE mean?
CADGE noun- A circular frame on which cadgers carry hawks for sale.
- To beg.
- To obtain something by wit or guile; to convince people to do something they might not normally do.
- To carry hawks and other birds of prey.
- To carry, as a burden.
- To hawk or peddle, as fish, poultry, etc.
- To intrude or live on another meanly; to beg.