Saucy always in Irish musical (10)
I believe the answer is:
irreverent
'saucy' is the definition.
(I know that saucy can be written as irreverent)
'always in irish musical' is the wordplay.
'always' becomes 'ever' (synonyms).
'in' means one lot of letters goes inside another.
'irish' becomes 'IR' (abbreviation).
'musical' becomes 'rent' (Rent is an example).
'ir'+'rent'='irrent'
'ever' placed inside 'irrent' is 'IRREVERENT'.
(Other definitions for irreverent that I've seen before include "Disrespectful" , "Cheeky" , "Not respectful" , "Impious" .)