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Prejudice will prosper, mostly, when cultivated by Congress (13)

Ross

I believe the answer is:

prepossession

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

'prejudice' is the definition.
(thesaurus)

'will prosper mostly when cultivated by congress' is the wordplay.
I cannot really see how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is present in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Hindu.
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