Bottle used as weapon by Clint Eastwood (6)

I believe the answer is:

magnum

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'bottle' is the definition.
(magnum is a kind of bottle)

I don't understand the remainder of the clue.

(Other definitions for magnum that I've seen before include "Large wine bottle" , "Two wine bottles in one" , "1 1/2 litre champagne bottle" , "Double-size wine bottle" , "Double-sized wine bottle" .)

I've seen this clue in The Guardian.

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