Small snake (5)
I believe the answer is:
adder
'small snake' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for adder that I've seen before include "Britain's only venomous snake" , "Snake (like an abacus?)" , "1 of 21 [SNAKES]" , "He's making an extension" , "Totter" .)