Her co-star upset big band (9)
I believe the answer is:
orchestra
'big band' is the definition.
'her co-star upset' is the wordplay.
'upset' is an anagram indicator.
'her'+'co'+'star'='hercostar'
'hercostar' anagrammed gives 'ORCHESTRA'.
(Other definitions for orchestra that I've seen before include "Musicians, maybe in theatre" , "Gamelan, for instance" , "Group of musical instrumentalists" , "those who provide the music" , "Musical ensemble" .)