A certainty on a tree, a source of quinine (8)

I believe the answer is:

cinchona

I'm a little stuck... Click here to teach me more about this clue!

'tree a source of quinine' is the definition.
I can't judge whether this definition defines the answer.

'a certainty on a' is the wordplay.
'a certainty' becomes 'cinch' (I've seen this before).
'cinch'+'on'+'a'='CINCHONA'

Can you help me to learn more?

(Other definitions for cinchona that I've seen before include "Medicinal bark" , "Tree, its bark for quinine" , "medicinal drug" .)

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

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