Six for every snake (5)
I believe the answer is:
viper
'snake' is the definition.
(I know that snake can be written as Viper)
'six for every' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'for every' becomes 'per' ('per' means 'for each' or 'for every').
'vi'+'per'='VIPER'
(Other definitions for viper that I've seen before include "spiteful person" , "poisonous type" , "crawler" , "A small poisonous snake" , "A poisonous snake or treacherous person" .)