The most common and most likely answer to this crossword clue is the 10 letter word CHANGELING
I've seen this crossword clue in Financial Times
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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 = changeling.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 CHANGELING mean?
CHANGELING noun- In pre-modern European mythology, an infant that was secretly exchanged for a mother's own baby by an evil creature. (In British, Irish and Scandinavian mythology the exchanged infants were thought to be those of fairies, sprites or trolls; in other places, they were ascribed to witches, devils, or demons.)
- An infant secretly exchanged with another infant by mistake or by human doing; swapling.
- An organism which can change shape to mimic others.
- A simpleton; an idiot.
- One apt to change; a waverer.