Copper not common in poison (6)
I believe the answer is:
curare
'poison' is the definition.
(curare is a poisonous plant)
'copper not common' is the wordplay.
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'not common' becomes 'rare' ('rare' is the opposite to 'common').
'cu'+'rare'='CURARE'
'in' acts as a link.
(Other definitions for curare that I've seen before include "Arrow poison" , "Paralysing plant poison" , "Poison used on darts" , "Poison put on arrows" , "Poison applied to darts" .)