Hair soap (7)
I believe the answer is:
shampoo
'hair soap' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for shampoo that I've seen before include "Liquid that goes to your head?" , "Nickname for champagne" , "Cleaner for locks" , "cleansing liquid" , "Oh, so Pam uses it on her hair?" .)