Substance used to treat Liberal Party Secretary (5)

I believe the answer is:

ldopa

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'substance used to treat' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'liberal party secretary' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'party' becomes 'do' (do can informally mean a party).
'secretary' becomes 'PA' (abbreviation for personal assistant).
'l'+'do'+'pa'='LDOPA'

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I've seen this clue in The Times.

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