A tax formerly fixed (3)

I believe the answer is:

set

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This might be a double definition.

'a tax' is the first definition.
(I know that set is a is a more specific form of the action of tax)

'formerly fixed' is the second definition.
'set' can be an answer for 'fixed' (both can mean decided or determined). I'm unsure of the 'formerly' bit.

Can you help me to learn more?

(Other definitions for set that I've seen before include "Scenery for play or film" , "Solidified" , "number of games" , "Congeal" , "See 16 Down" .)

I've seen this clue in the Evening Standard.

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