Drugged, but oddly steady after party (5)
I believe the answer is:
dosed
'drugged' is the definition.
('dose' can be a synonym of 'drug')
'oddly steady after party' is the wordplay.
'oddly' indicates alternate letters.
'after' means one lot of letters go next to another.
'party' becomes 'do' (do is a kind of party).
The alternate letters of 'steady' are 'sed'.
'sed' put after 'do' is 'DOSED'.
'but' acts as a link.
(Other definitions for dosed that I've seen before include "Gave medicine to" , "Treated with medicine" , "Given shot" , "Given medicine" , "Medicated" .)