The most common and most likely answer to this crossword clue is the 7 letter word ACIDIFY
A
C
I
D
I
F
Y
More clues leading to the result ACIDIFY
Buddy explains!

The straight part
We know that "to make tart" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to make tart = acidify.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 ACIDIFY mean?
ACIDIFY verb- To make something (more) acidic or sour; to convert into an acid.
- To neutralize alkalis, as to acidify sugar
- To sour, to embitter.