Free from bitterness (7)
I believe the answer is:
sweeten
'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" .)