Rattle her co-star with a lot of players (9)
I believe the answer is:
orchestra
'players' is the definition.
(group playing music)
'rattle her co-star with a lot of' is the wordplay.
'rattle' is an anagram indicator.
'with' becomes 'w' (short for with).
'a lot of' means to remove the last letter.
'her'+'co'+'star'+'w'='hercostarw'
'hercostarw' with its final letter taken off is 'hercostar'.
'hercostar' is an anagram of 'ORCHESTRA'.
(Other definitions for orchestra that I've seen before include "alternative to the organ" , "Other cars (anag.)" , "Large group of musicians" , "Theatre area for musicians" , "those who provide the music" .)