Before the final, rest a poor substitute (6)
I believe the answer is:
ersatz
'substitute' is the definition.
(I know that ersatz is a type of substitute)
'before the final rest a poor' is the wordplay.
'before' is a charade indicator (letters next to each other).
'the final' becomes 'z' (I am not sure about this - if you are sure you should believe this answer much more).
'poor' is an anagram indicator.
'rest'+'a'='resta'
'resta' anagrammed gives 'ersat'.
'z' after 'ersat' is 'ERSATZ'.
Can you help me to learn more?
(Other definitions for ersatz that I've seen before include "Artificial or inferior imitation" , "Made in imitation, from German" , "inferior type" , "Inferior substitute" , "'Artificial, not genuine (6)'" .)