Insect not found to contain copper (6)
I believe the answer is:
locust
'insect' is the definition.
(locust is a kind of insect)
'not found to contain copper' is the wordplay.
'not found' becomes 'lost' ('lost' is the opposite to 'found').
'to contain' means one lot of letters goes inside another.
'copper' becomes 'cu' (chemical symbol).
'lost' placed around 'cu' is 'LOCUST'.
(Other definitions for locust that I've seen before include "It strips plants" , "Ravenous insect" , "Destroyer" , "Voracious insect, travels in swarms" , "Swarming grasshopper" .)