One of the beer drinker's overheads? (5)

I believe the answer is:

froth

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'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I cannot understand how the remainder of the clue works.

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(Other definitions for froth that I've seen before include "Mass of small bubbles" , "Effervesce" , "Lather" , "Spume or foam" , "Small bubbles on a liquid" .)

I've seen this clue in the Evening Standard.

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