Six for every reptile (5)
I believe the answer is:
viper
'reptile' is the definition.
(I know that viper is a type of snake)
'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 "treacherous type" , "A poisonous snake or treacherous person" , "Venomous snake, person" , "spiteful person" , "Malicious person" .)