Embarrasses adult son taking party drug (7)
I believe the answer is:
abashes
'embarrasses' is the definition.
('abash' can be a synonym of 'embarrass')
'adult son taking party drug' is the wordplay.
'adult' becomes 'a' (abbreviation).
'son' becomes 's' (genealogical abbreviation for son).
'taking' indicates putting letters inside.
'party' becomes 'bash' (term for a party or celebration).
'drug' becomes 'e' (E is another name for ecstasy).
'a'+'s'='as'
'bash'+'e'='bashe'
'as' going around 'bashe' is 'ABASHES'.
(Other definitions for abashes that I've seen before include "Embarrasses" , "Causes embarrassment" .)