Musical instruction gets unintelligent one undone, confused (10)
I believe the answer is:
diminuendo
'musical instruction' is the definition.
(musical instruction to decrease in volume)
'unintelligent one undone confused' is the wordplay.
'unintelligent' becomes 'dim'.
'one' becomes 'i' (Roman numeral).
'confused' indicates anagramming the letters.
'undone' with letters rearranged gives 'nuendo'.
'dim'+'i'+'nuendo'='DIMINUENDO'
'gets' is the link.
(Other definitions for diminuendo that I've seen before include "Letting sound fade away" , "losing force" , "becoming quieter" , "volume is to be cut" , "Decrease in loudness" .)