At the end, notable player posed with unknown substitute (6)
I believe the answer is:
ersatz
'substitute' is the definition.
(I know that ersatz is a type of substitute)
'at the end notable player posed with unknown' is the wordplay.
'at the end' says to take the final letters.
'posed' becomes 'sat' (posed or sat for an artist).
'with' is a charade indicator (letters next to each other).
'unknown' becomes 'z' (algebra).
The final letters of 'notable player' are 'er'.
'er'+'sat'+'z'='ERSATZ'
(Other definitions for ersatz that I've seen before include "Artificial, imitation - from German" , "Substitute - fake" , "Mock" , "'Artificial, not genuine (6)'" , "Artificial or inferior imitation" .)