The most common and most likely answer to this crossword clue is the 9 letter word HUNDREDTH
I've seen this crossword clue in The Daily Telegraph
H
U
N
D
R
E
D
T
H
More clues leading to the result HUNDREDTH
Buddy explains!

The straight part
We know that "The last sheep back in the fold" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the last sheep back in the fold = hundredth.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 HUNDREDTH mean?
HUNDREDTH noun- The person or thing in the hundredth position.
- One of a hundred equal parts of a whole.
- The ordinal form of the number one hundred.