Calculator as a cub used (6)
I believe the answer is:
abacus
'calculator' is the definition.
(abacus is a kind of calculator)
'as a cub used' is the wordplay.
'used' indicates anagramming the letters.
'as'+'a'+'cub'='asacub'
'asacub' anagrammed gives 'ABACUS'.
(Other definitions for abacus that I've seen before include "calculator needed" , "Counting-bead device" , "not something we now count on" , "It might work out" , "Manual calculator" .)