The most common and most likely answer to this crossword clue is the 9 letter word SO-CALLED
I've seen this crossword clue in The Guardian
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More clues leading to the result SO-CALLED
Buddy explains!

The straight part
We know that "supposed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case supposed = so-called.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 SO-CALLED mean?
SO-CALLED adjective- So named; commonly called by such a name, often with a connotation that the item is not worthy of that name.
- So named; commonly called.