Start to smoke joint and smell clean hair (7)
I believe the answer is:
Here is my best explanation:
'clean hair' is the definition.
'start to smoke joint and smell' is the wordplay.
'start to' says to take the initial letters.
'joint' becomes 'ham' (ham is a joint of meat).
'and' is a charade indicator (letters next to each other).
'smell' becomes 'poo' (poo smell is a type of smell).
The first letter of 'smoke' is 's'.
(Other definitions for shampoo that I've seen before include "Hair-cleaning product", "hair-care product", "Would Mao shop for this cleaner?", "Apply liquid to hair ...", "hair treatment", "Hair wash preparation", "lather", "Wash hair", "Hair or carpet cleaner" and "Washing preparation (from Hindi)".)