Catch a cold in a public vehicle — 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)
'catch a cold in a public vehicle' is the wordplay.
'catch a cold' becomes 'ac' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'public vehicle' becomes 'bus' (I've seen this in another clue).
'a'+'bus'='abus'
'ac' put inside 'abus' is 'ABACUS'.
(Other definitions for abacus that I've seen before include "You could count on this once" , "What was counted on" , "that one can count on" , "this adds up" , "not something we now count on" .)