Some venomous snakes (6)
I believe the answer is:
adders
'some venomous snakes' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for adders that I've seen before include "Mathematical snakes" , "Dreads (anag) -- snakes" , "Totting 25 down?" , "Re Dad's snakes" , "at the zoo?" .)