Party attended by composer leaving new widow (7)
I believe the answer is:
dowager
'new widow' is the definition.
'dowager' can be an answer for 'widow' (term for an aristocratic widow). I'm unsure of the 'new' bit.
'party attended by composer' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'attended by composer' becomes 'wager' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'do'+'wager'='DOWAGER'
'leaving' acts as a link.
(Other definitions for dowager that I've seen before include "woman of property?" , "Widow inheriting husband's title or property" , "Wealthy old woman" , "Woman who holds title from dead husband" , "Widow with title or property from her husband" .)