Volume added to a top sound (5)
I believe the answer is:
valid
'sound' is the definition.
(similar in meaning)
'volume added to a top' is the wordplay.
'volume' becomes 'v' (abbreviation for volume in book titles).
'added to' means one lot of letters go next to another.
'top' becomes 'lid' (lid is a kind of top).
'v'+'a'+'lid'='VALID'
(Other definitions for valid that I've seen before include "Sound, well-based" , "Sound and to the point, also legal" , "Legitimate" , "Effective" , "Legally good" .)