Snake, police chief? (10)
I believe the answer is:
copperhead
'snake' is the definition.
(copperhead is a kind of snake)
'police chief?' is the wordplay.
'police' becomes 'copper' (I am not sure about this - if you are sure you should believe this answer much more).
'chief?' becomes 'head' ('head' can be a synonym of 'chief').
'copper'+'head'='COPPERHEAD'
(Other definitions for copperhead that I've seen before include "Policeman brains snake" , "Venomous, reddish-brown American snake" , "Highly venomous US or Australian snake" , "venomous sort" , "Venomous type" .)