We're told, ring for cosmetic treatment (4)
I believe the answer is:
peel
'cosmetic treatment' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'we're told ring' is the wordplay.
'we're told' shows a homophone (sound like).
'ring' becomes 'peal' (pealing is a kind of ringing).
'peal' is a homophone of 'PEEL'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for peel that I've seen before include "Remove skin from with blade" , "Shed skin" , "old prime minister" , "island" , "Outer fruit layer" .)