Her co-star reformed this classical band (9)
I believe the answer is:
orchestra
'classical band' is the definition.
'orchestra' can be an answer for 'band' (musical band). I am not certain of the 'classical' bit.
'her co-star reformed' is the wordplay.
'reformed' indicates an anagram.
'her'+'co'+'star'='hercostar'
'hercostar' anagrammed gives 'ORCHESTRA'.
'this' acts as a link.
Can you help me to learn more?
(Other definitions for orchestra that I've seen before include "A number of musicians" , "Large body of musicians" , "Large group of instrumentalists" , "Large group of players" , "Assembly of musicians as in pit" .)