Eccentric expert with marijuana (8)
I believe the answer is:
crackpot
'eccentric' is the definition.
(I know that crackpot is a type of eccentric)
'expert with marijuana' is the wordplay.
'expert' becomes 'crack' (as in 'a crack team').
'with' is a charade indicator (letters next to each other).
'marijuana' becomes 'pot' (slang term for marijuana).
'crack'+'pot'='CRACKPOT'
(Other definitions for crackpot that I've seen before include "Crazy or impractical person" , "Nutty" , "Foolish" , "Fruitcake, screwball" , "Idiot" .)