The most common and most likely answer to this crossword clue is the 3 letter word ARE
I've seen this crossword clue in Financial Times
A
R
E
Buddy explains!
The straight part
We know that "land measure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case land measure = are.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 ARE mean?
ARE noun- An accepted (but deprecated and rarely used) SI unit of area equal to 100 square metres, or a former unit of approximately the same extent. Symbol: a.