If you want medicine, ring me and call round (7)

I believe the answer is:

calomel

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'if you want medicine' is the definition.
I can't tell whether this definition defines the answer.

'ring me and call round' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'and' says to put letters next to each other.
'round' is an insertion indicator.
'o'+'me'='ome'
'ome' placed into 'call' is 'CALOMEL'.

Can you help me to learn more?

(Another definition for calomel that I've seen is " All come (anag) - purgative".)

I've seen this clue in the Evening Standard.

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