Classical piece (6)
(Other definitions for sonata that I've seen before include "A musical form" , "See 6" , "piece for instrumental soloist" , "musical performance" , "Musical composition for one instrument or two" .)
'classical piece' is the definition.
(I know that concerto is a classical composition)
This is the entire clue.
(Other definitions for concerto that I've seen before include "Orchestral piece" , "Solo/orchestra piece" , "Musical work" , "piece of music" , "Composition for orchestra in several movements" .)