Medical administrations second in activities (7)

I believe the answer is:

dosings

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'medical administrations' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'second in activities' is the wordplay.
'second' becomes 'sing' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'activities' becomes 'dos' (I've seen this in another clue).
'sing' placed into 'dos' is 'DOSINGS'.

Can you help me to learn more?

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

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