Boy at back tackling advanced piece of music (6)
I believe the answer is:
sonata
'piece of music' is the definition.
(type of musical composition)
'boy at back tackling advanced' is the wordplay.
'boy' becomes 'son' (male child).
'back' says the letters should be written in reverse.
'tackling' is an insertion indicator (to tackle is to take possession of).
'advanced' becomes 'a' (abbreviation).
'at' written backwards gives 'ta'.
'son'+'ta'='sonta'
'sonta' going around 'a' is 'SONATA'.
(Other definitions for sonata that I've seen before include "Eg, Beethoven's Kreutzer or Spring" , "Musical work for a solo instrument" , "Work for soloist" , "Musical piece" , "musical performance" .)