The most common and most likely answer to this crossword clue is the 4 letter word WEFT
I've seen this crossword clue in The Daily Telegraph
W
E
F
T
More clues leading to the result WEFT
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 = weft.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 WEFT mean?
WEFT noun- The horizontal threads that are interlaced through the warp in a woven fabric.
- The yarn used for the weft; the fill.
- (hairdressing) A hair extension that is glued directly to a person′s natural hair.
- Something cast away; a waif.