After drink, sat and hypothesised (8)
I believe the answer is:
supposed
'hypothesised' is the definition.
('suppose' can be a synonym of 'hypothesize')
'after drink sat' is the wordplay.
'after' means one lot of letters go next to another.
'drink' becomes 'sup' (I've seen this before).
'sat' becomes 'posed' ('pose' can be a synonym of 'sit').
'sup'+'posed' is 'SUPPOSED'.
'and' is the link.
(Other definitions for supposed that I've seen before include "Reckoned, took to be the case" , "So-called" , "Assumed - hypothetical" , "Imagined, presumed" , "Imagined; assumed" .)