Elizabeth, upcoming celebrity, an unknown quantity (7)
I believe the answer is:
zillion
'an unknown quantity' is the definition.
(large unspecified number)
'elizabeth upcoming celebrity' is the wordplay.
'elizabeth' becomes 'liz' (short for Elizabeth).
'upcoming' says the letters should be written backwards (in a down clue coming up is backwards).
'celebrity' becomes 'lion' (both can mean a celebrated person).
'liz' back-to-front is 'zil'.
'zil'+'lion'='ZILLION'
(Other definitions for zillion that I've seen before include "Lino Liz (anag.)" , "Unimaginably high number" , "Huge, unspecified figure" , "Great quantity" , "Mind-blowing number" .)