The most common and most likely answer to this crossword clue is the 6 letter word OPINED
I've seen this crossword clue in The Daily Telegraph
O
P
I
N
E
D
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 = opined.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 OPINED mean?
OPINED verb- To have or express an opinion; to state as an opinion; to suppose, consider (that).
- To give one's formal opinion (on or upon something).