The most common and most likely answer to this crossword clue is the 16 letter word GOOD-FOR-NOTHING
I've seen this crossword clue in Independent
G
O
O
D
-
F
O
R
-
N
O
T
H
I
N
G
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 = good-for-nothing.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 GOOD-FOR-NOTHING mean?
GOOD-FOR-NOTHING noun- A person of little worth or usefulness.
- Useless; worthless.