Denounce copper's inside expert (6)
I believe the answer is:
accuse
'denounce' is the definition.
(thesaurus)
'copper's inside expert' is the wordplay.
'copper's' becomes 'Cu's' (chemical symbol).
'inside' indicates putting letters inside.
'expert' becomes 'ace' (ace can informally mean very good).
'cus' inserted within 'ace' is 'ACCUSE'.
(Other definitions for accuse that I've seen before include "Charge with wrong-doing" , "Blame for a crime" , "From what Zola wrote" , "make charges" , "Level a charge against" .)