Cain's brother, one with a shrub (6)
I believe the answer is:
abelia
'shrub' is the definition.
(abelia is a kind of shrub)
'cain's brother one with a' is the wordplay.
'cain's brother' becomes 'abel' (biblical figure).
'one' becomes 'i' (Roman numeral).
'with' says to put letters next to each other.
'abel'+'i'+'a'='ABELIA'
(Other definitions for abelia that I've seen before include "Honeysuckle" , "shrub" , "plant" .)