Fizzes fresh sea-water, tungsten-free (7)
I believe the answer is:
aerates
'fizzes' is the definition.
The definition and answer can be both related to change as well as being verbs in their -s form.
Perhaps they are linked in a way I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for aerates that I've seen before include "Pumps carbon dioxide into" , "Brings gas" , "uses fork to do it?" , "Puts fizz into a drink" , "Adds carbon dioxide to liquid under pressure" .)