Latte art medium (4)
I believe the answer is:
'latte art medium' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for foam that I've seen before include "Fizz, froth" , "At the mouth, this may mean madness or extreme anger" , "Froth created by wave or soap" , "Mass of small bubbles" , "Suds" .)