A police vessel? (6)
I believe the answer is:
copper
I believe this is a double definition.
'a police' is the first definition.
(thesaurus)
'vessel?' is the second definition.
(container formerly used for laundry)
(Other definitions for copper that I've seen before include "Reddish metallic element" , "Ductile metal" , "Metal for slang policeman" , "Coin, policeman or metallic element" , "One who nicks" .)