Copper plate oxidised at the edges (4)
I believe the answer is:
plod
'copper' is the definition.
(both are slang terms for a police officer)
'plate oxidised at the edges' is the wordplay.
'plate' becomes 'pl' (used in citations of historical documents).
'at the edges' says to hollow out the word (remove centre letters).
'oxidised' with its centre removed is 'od'.
'pl'+'od'='PLOD'
(Other definitions for plod that I've seen before include "Walk heavily and wearily" , "Slow heavy walk" , "Walk with heavy steps" , "Walk heavily (like a policeman?)" , "Shamus, say" .)