Simple computational device - one with slide? (5)
I believe the answer is:
adder
I believe this clue is a double definition.
'simple computational device' is the first definition.
(device for adding numbers)
'one with slide?' is the second definition.
(as in a slide adder - device for basic addition)
(Other definitions for adder that I've seen before include "Dread (anag)" , "Viper common in northern Eurasia" , "Britain's only venomous snake" , "One creature" , "danger in the countryside" .)