The most common and most likely answer to this crossword clue is the 9 letter word INDENTURE
I've seen this crossword clue in The Guardian
I
N
D
E
N
T
U
R
E
More clues leading to the result INDENTURE
- Agreement to get tough, hiding depression
- One under ten confused by legal deed
- Working contract for stylish gnashers?
- Sort of apprenticeship plate on walls of inn
- ... in true fashion include study contract
- Home study gets true agreement
- Agreement at home — something taken out overnight?
- Service contract for one new set of teeth
Buddy explains!

The straight part
We know that "legal deed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case legal deed = indenture.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 INDENTURE mean?
INDENTURE noun- A contract which binds a person to work for another, under specified conditions, for a specified time (often as an apprentice).
- A document, written as duplicates separated by indentations, specifying such a contract.
- An indentation.
- To bind a person under such a contract.
- To indent; to make hollows, notches, or wrinkles in; to furrow.