The most common and most likely answer to this crossword clue is the 8 letter word THEORISE
I've seen this crossword clue in The Daily Telegraph
T
H
E
O
R
I
S
E
More clues leading to the result THEORISE
- Suppose the old revolt?
- Doctor is here to speculate on …
- Guess what he or I secretly collect!
- Speculate in article about knight breaking poet’s heart
- Article ‘Johnson taking Britain out’ written by European is conjecture
- Speculate — that makes the euro finally increase in value
- Suppose, taking helium in, to go up
- Guess the pay increase after middle of October
- Guess he will split Tories, making late switch
- Suppose duck has got into the tower
Buddy explains!

The straight part
We know that "Suppose" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case suppose = theorise.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 THEORISE mean?
THEORISE verb- To formulate a theory, especially about some specific subject.
- To speculate.