Finally prepared for postage before the final big rush (8)
I believe the answer is:
stampede
'big rush' is the definition.
'stampede' can be an answer for 'rush' (I've seen this before). I am not sure about the 'big' bit.
'finally prepared for postage before the final' is the wordplay.
'finally prepared for' becomes 'ed' (I am not sure about this - if you are sure you should believe this answer much more).
'postage' becomes 'stamp' (synonyms).
'before' is a charade indicator (letters next to each other).
'final' says to take the final letters.
The final letter of 'the' is 'e'.
'ed' put after 'stamp' is 'stamped'.
'stamped'+'e'='STAMPEDE'
Can you help me to learn more?
(Other definitions for stampede that I've seen before include "Panic-driven rush of herd of animals" , "Rampage" , "Panicky run" , "Panicked rush (of cattle)" , "Wild headlong rush" .)