A vehicle carrying a hundred? You can count on it! (6)
I believe the answer is:
abacus
'you can count on it' is the definition.
(an abacus is a tool for doing sums)
'a vehicle carrying a hundred?' is the wordplay.
'vehicle' becomes 'bus' (public transport vehicle).
'carrying' means one lot of letters goes inside another.
'hundred?' becomes 'c' (Roman numeral).
'a'+'c'='ac'
'bus' placed around 'ac' is 'bacus'.
'a'+'bacus'='ABACUS'
(Other definitions for abacus that I've seen before include "means of multiplying" , "frame of sorts" , "You could count on this once" , "calculator needed" , "It might work out" .)