Most dangerous to consume drug, one essentially causing forgetfulness (7)
I believe the answer is:
lethean
'essentially causing forgetfulness' is the definition.
I can't tell whether this defines the answer.
'most dangerous to consume drug one' is the wordplay.
'most dangerous' becomes 'lan' (I am not sure about this - if you are sure you should believe this answer much more).
'to consume' indicates putting letters inside.
'drug' becomes 'e' (E is another name for ecstasy).
'one' becomes 'the' (both can introduce a definite example of something).
'e'+'the'='ethe'
'lan' going around 'ethe' is 'LETHEAN'.
Can you help me to learn more?
(Other definitions for lethean that I've seen before include "Causing forgetfulness and oblivion" , "wiping memory" , "affecting memory" , "Preventing recall" , "Making one forget" .)