Philistine drinks a lot in elementary form (7)

I believe the answer is:

isotope

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'elementary form' is the definition.
(form of a chemical element)

I don't understand how the rest of the clue works.

(Other definitions for isotope that I've seen before include "One form of element, differing in its neutrons from another" , "Oops, tie up the atom" , "Form of atomic nucleus" , "O, is poet involved in chemical element?" , "Oo, spite! (anag.)" .)

I've seen this clue in The Guardian.

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