Large malt with a twist guzzled? Not half! (3,3)

I believe the answer is:

ill say

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'not half' is the definition.
(both can be used to express agreement)

I can't explain the rest of the clue.

(Other definitions for ill say that I've seen before include "Rather!" , "indeed!" , "expression of agreement" , "Injuries" , "I totally agree!" .)

I've seen this clue in The Telegraph.

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