Become less wrinkly (7)
I believe the answer is:
flatter
'become' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for flatter that I've seen before include "sugar" , "drier" , "Comparatively deflated" , "Fawn upon" , "Become" .)