Weak passage including unfinished term (6)
I believe the answer is:
watery
'weak' is the definition.
('watery' can be a synonym of 'weak')
'passage including unfinished term' is the wordplay.
'passage' becomes 'way' (I've seen this in other clues**).
'including' indicates putting letters inside.
'unfinished' means to remove the last letter.
'term' with its last letter taken away is 'ter'.
'way' enclosing 'ter' is 'WATERY'.
(Other definitions for watery that I've seen before include "(Of food) thin, tasteless" , "Insipid" , "Weak, too diluted" , "Dilute; weak, pale" , "Aqueous" .)