Style of music (6,5)
(Another definition for thrash metal that I've seen is " Type of music".)
'style of music' is the definition.
This is the entire clue.
(Other definitions for classical that I've seen before include "from an earlier time" , "Type of music as opposed to pop, jazz etc" , "Traditionally accepted" , "Relating to ancient Greek and Latin cultures" , "of Rome?" .)