Herb mixture contains a treatment for addiction (5)
I believe the answer is:
rehab
'treatment for addiction' is the definition.
(I've seen this before)
'herb mixture contains a' is the wordplay.
'mixture' is an anagram indicator.
'contains' indicates putting letters inside.
'herb' with letters rearranged gives 'rehb'.
'rehb' going around 'a' is 'REHAB'.
(Other definitions for rehab that I've seen before include "Treatment for, eg, addiction" , "Alcohol clinic" , "Restoration to health after, eg, addiction (colloq.)" , "Treatment for alcohol or drug dependence" , "Course of treatment for addicts" .)