Activate the gum, perhaps (7)
I believe the answer is:
moisten
'activate' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Perhaps you can see a link between them that I can't see?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for moisten that I've seen before include "Make damp, say lips with tongue" , "contrary to sixteen down [DEHYDRATE]!" , "water a bit" , "What one's eyes may do!" , "It's on me (anag)" .)