Beethoven's Seventh, a top sound (5)
I believe the answer is:
valid
'sound' is the definition.
(I know that valid is a type of sound)
'beethoven's seventh a top' is the wordplay.
'beethoven's seventh' becomes 'v' (7th letter of 'beethoven').
'top' becomes 'lid' (lid is a kind of top).
'v'+'a'+'lid'='VALID'
(Other definitions for valid that I've seen before include "Still legally acceptable" , "Legally good" , "Legitimate" , "Sound and logical, also legal" , "Effective" .)