Aircon in a double-decker? 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)
'aircon in a double-decker?' is the wordplay.
'aircon' becomes 'ac' (I can't justify this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'double-decker?' becomes 'bus' (synonyms).
'a'+'bus'='abus'
'ac' going within 'abus' is 'ABACUS'.
Can you help me to learn more?
(Other definitions for abacus that I've seen before include "summer once?" , "Old counting-frame" , "Counting machine" , "Manual calculator" , "this adds up" .)