Codeine source (5)
I believe the answer is:
opium
'codeine source' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for opium that I've seen before include "wars with China" , "Poppy-based drug" , "where horse originally came from?" , "Poppy drug" , "Dried juice of white poppy" .)