The most common and most likely answer to this crossword clue is the 7 letter word FILL-IN
I've seen this crossword clue in Financial Times
F
I
L
L
-
I
N
More clues leading to the result FILL-IN
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 = fill-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 FILL-IN mean?
FILL-IN noun- A temporary replacement for another, especially at a job.
- A substitution.
- An intermediate result that must be stored temporarily during the course of a sparse matrix computation.
- A question or puzzle in which one is expected to fill in a missing part of something.
- A musical embellishment (usually percussion) that is added to connect musical phrases.
- Something added to fill a gap.
- Something added to increase the size of something; padding or filler.
- A product category that is used to complete a range or variety of a product line.