Pure cocaine's initial rush (6)
I believe the answer is:
chaste
'pure' is the definition.
('chaste' can be similar in meaning to 'pure')
'cocaine's initial rush' is the wordplay.
'initial' indicates taking the first letters.
'rush' becomes 'haste' ('haste' can be a synonym of 'rush').
The first letter of 'cocaines' is 'c'.
'c'+'haste'='CHASTE'
(Other definitions for chaste that I've seen before include "Showing virtue of kind" , "Everlasting" , "Unsullied" , "Morally pure" , "'Celibate, virginal (6)'" .)