Rule out contralto appearing in opening passage (8)
I believe the answer is:
preclude
'rule out' is the definition.
(synonyms)
'contralto appearing in opening passage' is the wordplay.
'contralto' becomes 'c' (musical abbreviation).
'appearing in' means one lot of letters goes inside another.
'opening passage' becomes 'prelude' (introductory piece of music).
'c' going within 'prelude' is 'PRECLUDE'.
(Other definitions for preclude that I've seen before include "Prevent, prohibit" , "Rule out or eliminate" , "bar" , "Debar" , "'Prevent, forestall (8)'" .)