Drug has unusual chemical compound (6)
I believe the answer is:
potash
'chemical compound' is the definition.
(potash is a kind of chemical compound)
'drug has unusual' is the wordplay.
'drug' becomes 'pot' (slang term for cannabis).
'unusual' indicates an anagram.
'has' is an anagram of 'ash'.
'pot'+'ash'='POTASH'
(Other definitions for potash that I've seen before include "Potassium compound used as fertiliser" , "field dressing" , "feed in bed?" , "Powerful alkali used in gardening" , "Potassium compound used in agriculture and industry" .)