Partners attending opening of observatory with cocaine (4)
I believe the answer is:
snow
'cocaine' is the definition.
('snow' can be a synonym of 'cocaine')
'partners attending opening of observatory with' is the wordplay.
'partners' becomes 'sn'.
'attending' says to put letters next to each other.
'opening of' indicates taking the first letters.
The first letters of 'observatory with' is 'ow'.
'sn'+'ow'='SNOW'
(Other definitions for snow that I've seen before include "Frozen rain" , "that may come at Christmas" , "Precipitation falling as flakes" , "Wintry weather" , "Wintry stuff" .)