Summer snake (5)

I believe the answer is:

adder

This clue is a double definition.

'summer' is the first definition.
(someone who adds or does sums)

'snake?' is the second definition.
(type of snake)

(Other definitions for adder that I've seen before include "Mathematical reptile" , "British snake; summer" , "Reptilian" , "Venomous type" , "Type of snake" .)

I've seen this clue in The Guardian and the Evening Standard.

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