The most common and most likely answer to this crossword clue is the 6 letter word CUBITS
I've seen this crossword clue in The Daily Telegraph
C
U
B
I
T
S
More clues leading to the result CUBITS
Buddy explains!

The straight part
We know that "they were used as measures once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they were used as measures once = cubits.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 CUBITS mean?
CUBITS noun- Various former units of length notionally based on the distance from a grown man's elbow to his fingertips, standardized in different places and times at values between 35 and 60 cm.
- The ulna.