True gauge of what's left in the gin bottle? (6,5)

I believe the answer is:

spirit level

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'true gauge' is the definition.
'spirit level' can be an answer for 'gauge' (thesaurus). I'm unsure of the 'true' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for spirit level that I've seen before include "Means of checking horizontality" , "Tool for checking horizontality" , "Device" , "Amount of whisky, say" , "Horizontality checker with a bubble" .)

I've seen this clue in The Globe and Mail.

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