Rule out C found in musical work (8)
I believe the answer is:
preclude
'rule out' is the definition.
(I know that rule out can be written as preclude)
'c found in musical work' is the wordplay.
'found in' means one lot of letters goes inside another.
'musical work' becomes 'prelude' (brief piece of music).
'c' placed within 'prelude' is 'PRECLUDE'.
(Other definitions for preclude that I've seen before include "Obviate, prevent" , "Prevent, prohibit" , "bar" , "Debar" , "'Prevent, forestall (8)'" .)