Standard selection from Beethoven or Mozart (4)
I believe the answer is:
norm
'standard' is the definition.
(I know that norm is a type of standard)
'selection from beethoven or mozart' is the wordplay.
'selection from' indicates a hidden word.
'NORM' is hidden in the letters of 'beethoven or mozart'.
(Other definitions for norm that I've seen before include "Average result" , "'Standard, usual (4)'" , "Small boy" , "Standard amount or customary behaviour" , "Standard level of performance" .)