Five plus one for each adder (5)
I believe the answer is:
viper
'adder' is the definition.
(I know that adder can be written as viper)
'five plus one for each' is the wordplay.
'five' becomes 'V' (Roman numeral).
'plus' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'for each' becomes 'per' (per person).
'v'+'i'+'per'='VIPER'
(Other definitions for viper that I've seen before include "spiteful person" , "A small poisonous snake" , "treacherous type" , "Malicious person" , "poisonous type" .)