Copper's covering evidence (5)
I believe the answer is:
proof
'evidence' is the definition.
(proof is a kind of evidence)
'copper's covering' is the wordplay.
'copper' becomes 'p'.
'covering' becomes 'roof' (roof is a kind of covering).
'p'+'roof'='PROOF'
(Other definitions for proof that I've seen before include "Draft copy of printed text" , "Test" , "Evidence that establishes the truth" , "Exposition ending QED?" , "Author may have to check this" .)