After false bottom for case, and zip? (8)
I believe the answer is:
'zip?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'after false bottom for case' is the wordplay.
'after' becomes 'post' (prefix meaning after, eg post-war).
'false bottom for case' becomes 'code' (I am not sure about this - if you are sure you should believe this answer much more).
'and' acts as a link.
Can you help me to learn more?
(Other definitions for postcode that I've seen before include "Letters and digits to aid sorting mail" , "Indication of area in address" , "Conclusion of address" , "Address identifier" , "Eg, E-Nine-Eight One-T-T" .)