Contemptuous expression of son leaving party (3)
I believe the answer is:
bah
'contemptuous expression' is the definition.
(expression of disapproval, as in 'bah humbug!')
'son leaving party' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'leaving' is a deletion indicator.
'party' becomes 'bash' (term for a party or celebration).
'bash' with 's' taken out is 'BAH'.
'of' is the link.
(Other definitions for bah that I've seen before include "''... Humbug'' Scrooge's cry" , "Exclamation of disgust or annoyance" , "Appalling!" , "''...! Humbug!'' (Scrooge)" , "Comic expression of contempt" .)