The most common and most likely answer to this crossword clue is the 8 letter word STAND-IN
I've seen this crossword clue in The Guardian
S
T
A
N
D
-
I
N
More clues leading to the result STAND-IN
- Be candidate, popular, as deputy
- Substitute’s goals finally beat Germany at home
- Wrong about function taken by daughter substitute
- Substitute starts to seem trustworthy and will get elected
- Relief when fools get sent back and put inside
- Substitute joiner impressed by introduction of serious money
- Substitute some florist and inspector
- Invest and invest, investing reserve
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 = stand-in.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 STAND-IN mean?
STAND-IN noun- A person of similar size and shape to an actor who "stands in" for that actor during the lengthy process of setting up a shot, but who, unlike a double, does not appear in the film.
- A substitute.