Notice red-backed snake? (5)
I believe the answer is:
adder
'snake?' is the definition.
(type of snake)
'notice red-backed' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'backed' says the letters should be written in reverse.
'red' reversed gives 'der'.
'ad'+'der'='ADDER'
(Other definitions for adder that I've seen before include "Small poisonous snake, viper" , "Venomous type" , "accountant?" , "British snake; summer" , "Type of snake" .)