Mean criminal admitting copper's shrewdness (6)
I believe the answer is:
acumen
'shrewdness' is the definition.
(acumen can mean shrewdness or good judgment)
'mean criminal admitting copper's' is the wordplay.
'criminal' is an anagram indicator.
'admitting' is an insertion indicator (admit can mean to welcome in or accept).
'copper' becomes 'cu' (chemical symbol).
'mean' anagrammed gives 'amen'.
'amen' placed around 'cu' is 'ACUMEN'.
(Other definitions for acumen that I've seen before include "Penetrating insight" , "Kind of sagacity, good for business" , "Awareness" , "perspicacity" , "Gumption" .)