Irrelevant additional intelligence about hallucinatory drug (10)
I believe the answer is:
extraneous
'irrelevant' is the definition.
(similar in meaning)
'additional intelligence about hallucinatory drug' is the wordplay.
I cannot really see how this works, but
'additional' could be 'extra' (I've seen this before) and 'extra' is found within the answer.
'drug' could be 'e' (E is another name for ecstasy) and 'e' is found in the answer.
'intelligence' could be 'nous' (I've seen this before) and 'nous' is found in the remaining letters.
No letters remain.
This explanation may well be incorrect...
(Other definitions for extraneous that I've seen before include "Of external origin" , "Irrelevant; of outside origin" , "not essential" , "Not belonging" , "Irrelevant to the subject" .)