The most common and most likely answer to this crossword clue is the 5 plus 4 letter word LUNAR YEAR
I've seen this crossword clue in The Sunday Telegraph
L
U
N
A
R
Y
E
A
R
More clues leading to the result LUNAR YEAR
Buddy explains!
The straight part
We know that "some time on the moon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case some time on the moon = lunar year.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.