Levy: one million? Difficult question left incomplete (6)
I believe the answer is:
impose
'levy' is the definition.
('impose' can be a synonym of 'levy')
'one million? difficult question left incomplete' is the wordplay.
'one million?' becomes 'im' (I've seen this in other clues).
'difficult question' becomes 'poser' (I've seen this before).
'left incomplete' means to remove the last letter.
'poser' with its last letter removed is 'pose'.
'im'+'pose'='IMPOSE'
(Other definitions for impose that I've seen before include "Force (something) to be done or accepted" , "Obtrude, force (on)" , "As task, set" , "Exact or levy, say tax" , "Make demands of" .)