The most common and most likely answer to this crossword clue is the 13 letter word SUPPOSITIONAL
S
U
P
P
O
S
I
T
I
O
N
A
L
Buddy explains!
The straight part
We know that "presumed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case presumed = suppositional.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.