Leave out one note in introductory piece (8)
I believe the answer is:
preclude
'leave out' is the definition.
(thesaurus)
'one note in introductory piece' is the wordplay.
'one note' becomes 'c' (musical note).
'in' means one lot of letters goes inside another.
'introductory piece' becomes 'prelude' (I've seen this before).
'c' inserted into 'prelude' is 'PRECLUDE'.
(Other definitions for preclude that I've seen before include "Obviate, prevent" , "Rule out or eliminate" , "bar" , "Prevent, forestall the possibility of" , "Debar" .)