Further down, missing second snake (5)
I believe the answer is:
adder
'snake' is the definition.
(adder is a kind of snake)
'further down missing second' is the wordplay.
'further down' becomes 'sadder' (I've seen this before).
'missing' indicates named letters should be taken away.
'second' becomes 's' (common abbreviation).
'sadder' with 's' taken away is 'ADDER'.
(Other definitions for adder that I've seen before include "Something you have to reckon with" , "Venomous type" , "One contributing more" , "British snake; summer" , "Reptilian" .)