Inform hooligan about drug to choose (6,3)
I believe the answer is:
single out
'choose' is the definition.
(I know that single out is a type of choose)
'inform hooligan about drug' is the wordplay.
'inform' becomes 'sing' (I've seen this in other clues).
'hooligan' becomes 'lout' (both can mean an aggressive person).
'about' means one lot of letters goes inside another.
'drug' becomes 'e' (E is another name for ecstasy).
'lout' enclosing 'e' is 'leout'.
'sing'+'leout'='SINGLE OUT'
'to' is the link.
(Other definitions for single out that I've seen before include "Select from group" , "Pick" , "Choose" , "record on release" .)