The most common and most likely answer to this crossword clue is the 6 letter word CREWEL
I've seen this crossword clue in The Daily Telegraph
C
R
E
W
E
L
Buddy explains!

The straight part
We know that "Yarn" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case yarn = crewel.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 CREWEL mean?
CREWEL noun- Worsted yarn, slackly twisted, used for embroidery.
- To make (embroidery) using the crewel method.