Snake seen in summer? (5)
I believe the answer is:
adder
I believe this is a double definition.
'snake' is the first definition.
(type of snake)
'summer?' is the second definition.
(someone who adds or does sums)
'seen in' is the link.
(Other definitions for adder that I've seen before include "Venomous sort" , "Reptilian" , "danger in the countryside" , "Dread (anag)" , "a poisonous type" .)