Supposed visitor could be picked up (7)
I believe the answer is:
guessed
'supposed' is the definition.
('guess' can be a synonym of 'suppose')
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for guessed that I've seen before include "perhaps got in one" , "Estimated correctly" , "was a speculator" , "Thought" , "Estimated, reckoned" .)