Policeman boss keeps a serpent (10)
I believe the answer is:
copperhead
'serpent' is the definition.
(copperhead is a kind of serpent)
'policeman boss keeps a' is the wordplay.
'policeman' becomes 'cop' (cop is a kind of policeman).
'boss' becomes 'head' (synonyms).
'keeps' means one lot of letters goes inside another.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'cop'+'head'='cophead'
'cophead' going around 'per' is 'COPPERHEAD'.
(Other definitions for copperhead that I've seen before include "Venomous, reddish-brown American snake" , "Highly venomous US or Australian snake" , "Reptile" , "Venomous type" , "Policeman brains snake" .)