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" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">classical piece</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that concerto is a classical composition)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)