Snake seen in summer? (5)
I believe the answer is:
adder
I believe this clue 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' acts as a link.
(Other definitions for adder that I've seen before include "Venomous sort" , "Reptilian" , "danger in the countryside" , "Dread (anag)" , "a poisonous type" .)