Watch demonstration including commercial for beauty aid (9)
I believe the answer is:
eyeshadow
'beauty aid' is the definition.
(I've seen this before)
'watch demonstration including commercial' is the wordplay.
'watch' becomes 'eye' (eye can mean to watch or look at).
'demonstration' becomes 'show' (show is a kind of demonstration).
'including' is an insertion indicator.
'commercial' becomes 'ad' (abbreviation for advertisement).
'show' placed around 'ad' is 'shadow'.
'eye'+'shadow'='EYESHADOW'
'for' is the link.
(Other definitions for eyeshadow that I've seen before include "cosmetic" , "part of make-up" , "beauty aid" , "used to put dark side into orbit" , "Kind of warpaint?" .)