Free from bitterness (7)

I believe the answer is:

sweeten

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'free from bitterness' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Maybe there's a link between them I don't understand?

This is all the clue.

Can you help me to learn more?

(Other definitions for sweeten that I've seen before include "temper" , "We teens! (anag)" , "Make sugary or saccharine" , "Add sugar etc" , "Appease" .)

I've seen this clue in The Irish Independent and The Globe and Mail.

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