Draws tops over parasitic plant (9)

I believe the answer is:

rafflesia

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'parasitic plant' is the definition.
(I've seen this before)

'draws tops over' is the wordplay.
'draws' becomes 'raffles' (I've seen this before).
'tops over' becomes 'ia' (I can't explain this - if you can you should believe this answer much more).
'raffles'+'ia'='RAFFLESIA'

Can you help me to learn more?

(Other definitions for rafflesia that I've seen before include "parasitic plant" , "Plant with a large malodorous bloom" , "Plant smelling of carrion; flies afar (anag.)" .)

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

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