Old class. A measure of whisky for medicine (5,4)

I believe the answer is:

fifth form

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'a measure of whisky for medicine' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'old class' is the wordplay.
'old' becomes 'fifth' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'class' becomes 'form' (synonyms).
'fifth'+'form'='FIFTH FORM'

Can you help me to learn more?

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

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