Free once, then fixed price (9)

I believe the answer is:

exonerate

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'free' is the definition.
(thesaurus)

'once then fixed price' is the wordplay.
'once' becomes 'ex'.
'then fixed' becomes 'one' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'price' becomes 'rate' (I've seen this in other clues).
'ex'+'one'+'rate'='EXONERATE'

Can you help me to learn more?

(Other definitions for exonerate that I've seen before include "Clear of blame" , "Discharge" , "let off the hook" , "Acquit" , "Declare to be blameless (9)" .)

I've seen this clue in The Independent.

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