As said to have been levied on wealthy knights? (6)

I believe the answer is:

surtax

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'as' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

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(Other definitions for surtax that I've seen before include "Additional levy" , "extra duty" , "Additional charge" .)

I've seen this clue in the Evening Standard.

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