Rule out dropping contralto for opening (7)
I believe the answer is:
prelude
'opening' is the definition.
'rule out dropping contralto' is the wordplay.
'rule out' becomes 'preclude' ('preclude' can be a synonym of 'rule out').
'dropping' is a deletion indicator.
'contralto' becomes 'c' (musical abbreviation).
'preclude' with 'c' taken out is 'PRELUDE'.
'for' is the link.
(Other definitions for prelude that I've seen before include "Introductory passage" , "Introductory part, say of poem" , "Preamble" , "(Musical) introduction" , "Curtain-raiser" .)