Mathematical snakes (6)
I believe the answer is:
adders
'mathematical snakes' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for adders that I've seen before include "Dreads (anag) -- snakes" , "sum? We do" , "People doing sums" , "Snakes are totters" , "Re Dad's snakes" .)