Gutted, Edgar was on the bench, perhaps an unknown substitute (6)
I believe the answer is:
ersatz
'substitute' is the definition.
(synonyms)
'gutted edgar was on the bench perhaps an unknown' is the wordplay.
'gutted' means to remove the middle letters (to gut something is to remove its insides).
'was on the bench perhaps' becomes 'sat'.
'an unknown' becomes 'z' (algebra).
'edgar' with its middle removed is 'er'.
'er'+'sat'+'z'='ERSATZ'
(Other definitions for ersatz that I've seen before include "Made in imitation - in Germany" , "Substitute - fake" , "Not real or genuine" , "Artificial, imitation - from German" , "Mock" .)