Copper's covering evidence (5)
'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).
(Other definitions for proof that I've seen before include "Specimen", "Conclusive demonstration", "may need amending", "a writer may check", "Render impervious (to)".)