Choristers with grant to reform music group (6,9)
I believe the answer is:
string orchestra
'music group' is the definition.
(I know that string orchestra is a type of orchestra)
'choristers with grant to reform' is the wordplay.
'with' means one lot of letters go next to another.
'to reform' is an anagram indicator (letters in a new form).
'choristers' put after 'grant' is 'grantchoristers'.
'grantchoristers' with letters rearranged gives 'STRING ORCHESTRA'.
(Other definitions for string orchestra that I've seen before include "group" , "Players" .)