Copper in peril -- a criminal's eccentric (8)
I believe the answer is:
peculiar
'eccentric' is the definition.
(thesaurus)
'copper in peril a criminal's' is the wordplay.
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'in' is an insertion indicator.
'criminal's' indicates an anagram (I've seen 'criminal' mean this).
'peril'+'a'='perila'
'perila' with letters rearranged gives 'peliar'.
'cu' inserted into 'peliar' is 'PECULIAR'.
(Other definitions for peculiar that I've seen before include "Characteristic" , "Extraordinary" , "Strange or eccentric, quite odd" , "Belonging exclusively to, person or place" , "sick" .)