Expression of self, one finally achieved with post-death message (3,4)
I believe the answer is:
ego trip
'expression of self' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'one finally achieved with post-death message' is the wordplay.
'finally' indicates one should take the final letters.
'achieved' becomes 'got' (get can mean to achieve).
'with post death message' becomes 'rip' (I can't justify this - if you can you should believe this answer much more).
The final letter of 'one' is 'e'.
'e'+'got'+'rip'='EGO-TRIP'
Can you help me to learn more?
(Other definitions for ego trip that I've seen before include "Act of self-glorification" , "Vainglorious course of action" , "Action to inflate one's sense of self-importance" , "Booster of self-image" , "Self-importance booster" .)