Chief constable is a venomous creature (10)
I believe the answer is:
copperhead
I believe this clue is a double definition.
'chief constable' is the first definition.
(whimsically, the head of all 'coppers')
'venomous creature' is the second definition.
(venomous snake)
'is a' acts as a link.
(Other definitions for copperhead that I've seen before include "Venomous, reddish-brown American snake" , "Policeman brains snake" , "Reptile" , "Venomous type" , "Highly venomous US or Australian snake" .)