Acne treatment is a punishment (4)
I believe the answer is:
cane
'punishment' is the definition.
(I've seen this in other clues)
'acne treatment' is the wordplay.
'treatment' indicates an anagram (letters treated or processed differently).
'acne' anagrammed gives 'CANE'.
'is a' is the link.
(Other definitions for cane that I've seen before include "Slender stick" , "A slender rod, perhaps for punishment" , "Type of sugar; beat" , "Strong, flexible stem" , "Eg, bamboo; punish" .)