Coke and cannabis for a crank (8)
I believe the answer is:
crackpot
'for a crank' is the definition.
('crackpot' can be a synonym of 'crank')
'coke and cannabis' is the wordplay.
'coke' becomes 'crack' (crack is a kind of coke).
'and' means one lot of letters go next to another.
'cannabis' becomes 'pot' (synonyms).
'crack'+'pot'='CRACKPOT'
(Other definitions for crackpot that I've seen before include "Crazy or impractical person" , "Bats" , "Eccentric; impractical" , "Fruitcake, screwball" , "Foolish" .)