Pest never found when copper enters (6)
I believe the answer is:
locust
'pest never' is the definition.
I can't tell whether this definition defines the answer.
'when copper enters' is the wordplay.
'when' becomes 'lost' (I can't justify this - if you can you should give a lot more credence to this answer).
'copper' becomes 'Cu' (chemical symbol).
'enters' means one lot of letters goes inside another.
'lost' placed around 'cu' is 'LOCUST'.
'found' is the link.
Can you help me to learn more?
(Other definitions for locust that I've seen before include "Predatory grasshopper that swarms" , "It strips plants" , "beast resulting in destruction" , "Destructive tropical grasshopper" , "Large insect" .)