Snake found in Summer? (5)
I believe the answer is:
adder
I believe this clue is a double definition.
'snake' is the first definition.
(I know that adder is a type of viper)
'summer?' is the second definition.
(someone who adds or does sums)
'found in' acts as a link.
(Other definitions for adder that I've seen before include "accountant?" , "Mathematical reptile" , "1 of 21 [SNAKES]" , "Dread (anag)" , "Venomous sort" .)