Little bit of dope which you can smoke to get high (4)
I believe the answer is:
weed
'which you can smoke to get high' is the definition.
(slang term for marijuana)
'little bit of dope' is the wordplay.
'little' becomes 'wee' (similar in meaning).
'bit of' indicates taking the first letters.
The first letter of 'dope' is 'd'.
'wee'+'d'='WEED'
(Other definitions for weed that I've seen before include "Marijuana (slang)" , "gardening" , "Hoe to remove it" , "Troublesome plant" , "Unwanted plant - cannabis" .)