Head on a glass of beer? (6)

I believe the answer is:

noggin

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'head' is the definition.
(I know that noggin is a type of human head)

I don't understand the remainder of the clue.

(Other definitions for noggin that I've seen before include "Small measure of spirits or slangy head" , "Gill usually" , "Peg" , "Small jug of 19, perhaps" , "Small quantity of spirits" .)

I've seen this clue in the Sydney Morning Herald.

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