Expression of contempt when son leaves party (3)
I believe the answer is:
bah
'expression of contempt' is the definition.
(I've seen this before)
'son leaves party' is the wordplay.
'son' becomes 's'.
'leaves' suggests deleting specific letters.
'party' becomes 'bash' (bash is a kind of party).
'bash' with 's' taken out is 'BAH'.
'when' acts as a link.
(Other definitions for bah that I've seen before include "''... Humbug'' Scrooge's cry" , "''...! Humbug!'' (Scrooge)" , "Comic expression of contempt" , "Exclamation of contempt or disgust - Scrooge used to say it" , "Expression of disgust or contempt" .)