Commercial for the German snake (5)
I believe the answer is:
adder
'snake' is the definition.
(I know that adder is a type of viper)
'commercial for the german' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'for' means one lot of letters go next to another.
'the german' becomes 'der' ('the' in German).
'ad'+'der'='ADDER'
(Other definitions for adder that I've seen before include "Type of snake" , "a poisonous type" , "Snake (like an abacus?)" , "danger in the countryside" , "Totting snake" .)