Uncommon poison copper overlooked (4)
I believe the answer is:
rare
'uncommon' is the definition.
(similar in meaning)
'poison copper overlooked' is the wordplay.
'poison' becomes 'curare' (I've seen this before).
'copper' becomes 'cu' (chemical symbol).
'overlooked' suggests deleting specific letters.
'curare' with 'cu' taken away is 'RARE'.
(Other definitions for rare that I've seen before include "Very infrequent or scarce" , "Not well done!" , "Rear up, being scarce" , "Meat that's lightly cooked" , "Excellent; uncommon" .)