Clean ersatz stool (7)
I believe the answer is:
shampoo
'clean' is the definition.
(to shampoo your hair is to clean it)
'ersatz stool' is the wordplay.
'ersatz' becomes 'sham' (both can mean false).
'stool' becomes 'poo' (both can mean excrement).
'sham'+'poo'='SHAMPOO'
(Other definitions for shampoo that I've seen before include "treat shock?" , "Liquid cleaner" , "Hair or carpet cleaner" , "Washer for locks" , "people get set after it" .)