The most common and most likely answer to this crossword clue is the 3 letter word SEW
S
E
W
More clues leading to the result SEW
Buddy explains!

The straight part
We know that "Darn" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case darn = sew.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 SEW mean?
SEW verb- To use a needle to pass thread repeatedly through (pieces of fabric) in order to join them together.
- To use a needle to pass thread repeatedly through pieces of fabric in order to join them together.
- Followed by into: to enclose by sewing.
- To drain the water from.
- Of a ship, to be grounded.