Her co-star played in a musical group (9)
I believe the answer is:
orchestra
'a musical group' is the definition.
(orchestra is a kind of musical group)
'her co-star played' is the wordplay.
'played' indicates an anagram.
'her'+'co'+'star'='hercostar'
'hercostar' anagrammed gives 'ORCHESTRA'.
'in' acts as a link.
(Other definitions for orchestra that I've seen before include "Large band playing classical music" , "Large group of instrumentalists" , "Musical ensemble" , "eg Halle" , "Large band of players" .)