Old calculator a conductor originally used in a double-decker? (6)
I believe the answer is:
abacus
'old calculator' is the definition.
(I've seen this before)
'a conductor originally used in a double-decker?' is the wordplay.
'originally' suggests taking the first letters.
'used in' is an insertion indicator.
'double-decker?' becomes 'bus' ('bus' can be a synonym of 'double-decker').
The first letter of 'conductor' is 'c'.
'a'+'c'='ac'
'a'+'bus'='abus'
'ac' placed within 'abus' is 'ABACUS'.
(Other definitions for abacus that I've seen before include "Beaded counting frame" , "one counts on it" , "An old counting frame with beads" , "Old counting-frame" , "Old counter" .)