Be against attitude presented by musical work (6)
I believe the answer is:
oppose
'be against' is the definition.
(synonyms)
'attitude presented by musical work' is the wordplay.
'attitude' becomes 'pose' (attitude can mean a pose or stance).
'presented by' says to put letters next to each other.
'musical work' becomes 'op' (abbreviation for opus).
'pose' put after 'op' is 'OPPOSE'.
(Other definitions for oppose that I've seen before include "Act against" , "Resist, contend with" , "Bar" , "Resist (someone)" , "Argue against" .)