Pest missing when copper is introduced (6)
I believe the answer is:
locust
'pest' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'missing when copper is introduced' is the wordplay.
'missing when' becomes 'lost' (similar in meaning. I am not sure about the 'when' bit.).
'copper' becomes 'cu' (chemical symbol).
'is introduced' indicates putting letters inside.
'lost' placed around 'cu' is 'LOCUST'.
Can you help me to learn more?
(Other definitions for locust that I've seen before include "beast resulting in destruction" , "Voracious African or Asian grasshopper" , "Sort of migratory grasshopper" , "Sturdy, resilient" , "Clouts (anag)" .)