Choir's foremost mature composer (4)
I believe the answer is:
cage
'composer' is the definition.
(I've seen this before)
'choir's foremost mature' is the wordplay.
'foremost' says to take the initial letters.
'mature' becomes 'age' (synonyms).
The first letter of 'choirs' is 'c'.
'c'+'age'='CAGE'
(Other definitions for cage that I've seen before include "he used to write down scores" , "Form of incarceration" , "Place of confinement" , "Confine -- US composer, d. 1992" , "Eg, aviary" .)