Fake, largely inferior, hair preparation (7)
I believe the answer is:
shampoo
'hair preparation' is the definition.
(preparation applied to the hair)
'fake largely inferior' is the wordplay.
'fake' becomes 'sham' (synonyms).
'largely' means to remove the last letter.
'inferior' becomes 'poor' (similar in meaning).
'poor' with its last letter removed is 'poo'.
'sham'+'poo'='SHAMPOO'
(Other definitions for shampoo that I've seen before include "preparation for cleaning locks" , "16 21 [INDIAN IMPORT]" , "Oh, so Pam uses it on her hair?" , "Would Mao shop for this cleaner?" , "Cleaning fluid" .)