Mathematical snake (5)
I believe the answer is:
adder
'snake' is the definition.
(adder is a kind of snake)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for adder that I've seen before include "An arithmetic snake" , "exhibit in the reptile house" , "accountant?" , "on country walks?" , "Snake (like an abacus?)" .)