Opera venue: it involves contralto in singing without words (2,5)
I believe the answer is:
la scala
'opera venue' is the definition.
(Milanese opera house)
'it involves contralto in singing without words' is the wordplay.
'it' becomes 'sa' (old-fashioned abbreviation for sex appeal).
'involves' is an insertion indicator.
'contralto' becomes 'c' (musical abbreviation).
'in' is an insertion indicator.
'singing without words' becomes 'lala' (syllables which might be sung).
'sa' placed around 'c' is 'sca'.
'sca' going into 'lala' is 'LA SCALA'.
(Other definitions for la scala that I've seen before include "Italian opera house" , "noted venue?" , "music centre" , "where Toscanini was musical director" , "place to sing" .)