Drug is found following arrest behind prison (8)
I believe the answer is:
cannabis
'drug' is the definition.
(cannabis is a kind of drug)
'following arrest behind prison' is the wordplay.
'following arrest' becomes 'nabis' (I can't justify this - if you can you should believe this answer much more).
'behind' means one lot of letters go next to another.
'prison' becomes 'can' (the can is American slang for prison).
'nabis' put after 'can' is 'CANNABIS'.
'is found' is the link.
Can you help me to learn more?
(Other definitions for cannabis that I've seen before include "Hemp plant for making intoxicant drug" , "Soft drug" , "Ban Incas from using this drug" , "Marijuana" , "Strangely, Incas ban this drug" .)