Surmised (7)
'surmised' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for guessed that I've seen before include "Conjectured" , "Reckoned, judged" , "Thought" , "Estimated, reckoned" , "was a speculator" .)
(Other definitions for opined that I've seen before include "Conjectured" , "Gave as personal view" , "Gave as view or thought" , "Expressed as one's view" , "Gave as one's view" .)