Pure heroin snorted by actors with MDMA (6)
I believe the answer is:
Here is my best explanation:
'pure' is the definition.
('chaste' can be similar in meaning to 'pure')
'heroin snorted by actors with mdma' is the subsidiary indication.
I cannot quite understand how this works, but
'heroin' could be 'h' ('H' can be a synonym of 'heroin') and 'h' is present in the answer.
'actors' could be 'cast' and 'cast' is present in the remaining letters.
A single letter 'e' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for chaste that I've seen before include "Restrained, modest", "Virgin", "Modest and virtuous", "Pure, modest", "Pure and virginal", "Decorous", "avoiding relations", "Cheats (anag.)", "Decent" and "Unsullied".)