A betting agency runs popular drug (7)

I believe the answer is:

atabrin

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'drug' is the definition.
I know nothing about this answer so I can't tell whether this works.

'a betting agency runs popular' is the wordplay.
'betting agency' becomes 'tab' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'runs' becomes 'r' (cricket abbreviation).
'popular' becomes 'in' ('in' can mean trendy).
'a'+'tab'+'r'+'in'='ATABRIN'

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

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