Poison not often seen by copper (6)
I believe the answer is:
curare
'poison' is the definition.
(curare is a poisonous plant)
'not often seen by copper' is the wordplay.
'not often seen' becomes 'rare' (I've seen this in another clue).
'by' means one lot of letters go next to another.
'copper' becomes 'cu' (Cu is the chemical symbol for copper).
'rare' after 'cu' is 'CURARE'.
(Other definitions for curare that I've seen before include "Poison used on darts" , "Paralysing plant poison" , "Poison put on arrows" , "Arrow poison" , "Poison applied to darts" .)