Treating acne as a punishment (4)
I believe the answer is:
cane
'a punishment' is the definition.
(I've seen this before)
'treating acne' is the wordplay.
'treating' indicates an anagram.
'acne' anagrammed gives 'CANE'.
'as' is the link.
(Other definitions for cane that I've seen before include "A stick to lean on" , "A light rod once used for punishment" , "Slender stem of bamboo etc" , "Stick; thrash" , "A slender rod, perhaps for punishment" .)