The most common and most likely answer to this crossword clue is the 6 letter word ERSATZ
I've seen this crossword clue in The Times
E
R
S
A
T
Z
More clues leading to the result ERSATZ
- Spurious appeal lodged by Physics unit, commonly
- Substitute possessed by Rangers at Zagreb
- Substitute some tigers at zoo
- Gutted, Edgar was on the bench, perhaps an unknown substitute
- Substitute monarch posed with Zulu
- Phoney OT book, in short, about son lying in grass
- Phoney European hotel with appeal for Irish leader
- Mock workers at Zurich houses
- Artificial product of keepers at zoo
- Bogus bunch of bankers at Zurich
Buddy explains!

The straight part
We know that "Substitute" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case substitute = ersatz.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ERSATZ mean?
ERSATZ noun- Something made in imitation; an effigy or substitute
- Made in imitation; artificial, especially of a poor quality