The most common and most likely answer to this crossword clue is the 6 letter word DEBTOR
I've seen this crossword clue in The Guardian
D
E
B
T
O
R
More clues leading to the result DEBTOR
- Ruin part of garden when reversing? I’m expected to pay
- One has an obligation to gather up rubbish and litter
- He is liable to get dressed up without a shirt
- One owes money due but with order empty
- He must have been given credit for something
- Over time, dressed up somebody in red
- One with obligation to run after posh girl
- One yet to settle orbited round leaving island
- One who owes Robert was beheaded, mutilated but dead earlier
- One owing money in bar about to accept reckless bet
Buddy explains!

The straight part
We know that "One has an obligation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one has an obligation = debtor.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 DEBTOR mean?
DEBTOR noun- A person or firm that owes money; one in debt; one who owes a debt
- One who owes another anything, or is under obligation, arising from express agreement, implication of law, or principles of natural justice, to pay money or to fulfill some other obligation; in bankruptcy or similar proceedings, the person who is the subject of the proceeding.