Acne treatment is a punishment (4)
I believe the answer is:
cane
'punishment' is the definition.
(I've seen this before)
'acne treatment' is the wordplay.
'treatment' is an anagram indicator.
'acne' is an anagram of '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" .)