Conjecturing American will support drink stance? Not entirely (12)

I believe the answer is:

suppositious

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'conjecturing american will support drink stance? not entirely' is the definition.
I can't judge whether this defines the answer.

'american will support drink stance? not entirely' is the wordplay.
'american will support' becomes 'supus' (I am not sure about this - if you are sure you should believe this answer much more).
'drink' means one lot of letters goes inside another.
'stance?' becomes 'position' (stance is a kind of position).
'not entirely' means to remove the last letter (not all of the word).
'position' with its final letter taken away is 'positio'.
'supus' enclosing 'positio' is 'SUPPOSITIOUS'.

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I've seen this clue in The Independent.

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