Upset, and not to perform musical piece (5)
I believe the answer is:
rondo
'musical piece' is the definition.
(type of musical piece)
'upset and not to perform' is the wordplay.
'upset' says the letters should be written in reverse.
'and not' becomes 'nor' ('nor' means 'and also not').
'to perform' becomes 'do' (both can mean complete an action).
'nor' written backwards gives 'ron'.
'ron'+'do'='RONDO'
(Other definitions for rondo that I've seen before include "Musical form with returning theme" , "Donor (anag) - piece of music" , "Musical piece with a recurring theme" , "Repetitive musical form" , "part of a sonata" .)