Shrub found in Colorado and California (4)
I believe the answer is:
coca
'shrub' is the definition.
(I know that coca is a shrub)
'colorado and california' is the wordplay.
'colorado' becomes 'co'.
'and' says to put letters next to each other.
'california' becomes 'ca'.
'co'+'ca'='COCA'
'found in' is the link.
(Other definitions for coca that I've seen before include "Drug-yielding shrub" , "South American plant from which a narcotic drug is obtained" , "Source of drug" , "Leaves for snow" , "Narcotic shrub" .)