Unconventional star upset before hit national shows (8)
I believe the answer is:
antihero
'unconventional star' is the definition.
'upset before hit national shows' is the wordplay.
'upset' says the letters should be written backwards.
'shows' says the answer is hidden in the clue.
'orehitna' is hidden in the letters of 'before hit national'.
'orehitna' written backwards gives 'ANTIHERO'.
(Other definitions for antihero that I've seen before include "Less than admirable character" , "baddie" , "ignoble character" , "principal character lacking noble qualities" , "Central character lacking traditional courageous attributes" .)