Dickens protagonist rebuffs field officer (6)
I believe the answer is:
copper
'officer' is the definition.
(copper is a kind of officer)
'dickens protagonist rebuffs field' is the wordplay.
'dickens protagonist' becomes 'copperfield' (David Copperfield).
'rebuffs' is a deletion indicator.
'copperfield' with 'field' taken away is 'COPPER'.
(Other definitions for copper that I've seen before include "Metal for slang policeman" , "Police officer (slang)" , "detector?" , "Reddish metal of a penny" , "Ductile metal" .)