The most common and most likely answer to this crossword clue is the 4 plus 4 letter word BOOK DEBT
B
O
O
K
D
E
B
T
Buddy explains!
The straight part
We know that "Amount owed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case amount owed = book debt.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.