Copper seldom used muscle relaxant (6)
I believe the answer is:
curare
'muscle relaxant' is the definition.
(I've seen this in another clue)
'copper seldom' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'seldom' becomes 'rare' (I've seen this in another clue).
'cu'+'rare'='CURARE'
'used' acts as a link.
(Other definitions for curare that I've seen before include "Arrow poison" , "Poison used on darts" , "Paralysing plant poison" , "Poison applied to darts" , "Poison put on arrows" .)