Sliding calculator? (5)
I believe the answer is:
adder
I believe this clue is a double definition.
'sliding' is the first definition.
(I've seen this before)
'calculator?' is the second definition.
(I've seen this before)
(Other definitions for adder that I've seen before include "Viper common in northern Eurasia" , "Snake (like an abacus?)" , "Something you have to reckon with" , "One contributing more" , "danger in the countryside" .)