This snake counts? (5)
I believe the answer is:
adder
'this snake counts?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for adder that I've seen before include "Reptilian" , "Something you have to reckon with" , "Mathematical reptile" , "Venomous type" , "Poisonous Brit. snake" .)