Where some may learn of drug (speed) (4)
I believe the answer is:
eton
'where some may learn' is the definition.
(English public school)
'drug speed' is the wordplay.
'drug' becomes 'e' (e is another name for ecstasy).
'speed' becomes 'ton' (I've seen this before).
'e'+'ton'='ETON'
'of' acts as a link.
(Other definitions for eton that I've seen before include "learn here?" , "Posh British public school" , "Berkshire public school" , "Town on the Thames noted for its school" , "Public school, established in 1440" .)