The most common and most likely answer to this crossword clue is the 8 letter word SURMISED
I've seen this crossword clue in The Guardian
S
U
R
M
I
S
E
D
Buddy explains!
The straight part
We know that "Thought/to have speculated" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thought/to have speculated = surmised.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 SURMISED mean?
SURMISED verb- To imagine or suspect; to conjecture; to posit with contestable premises.