Features of the moon rock at first put in boxes (7)
I believe the answer is:
craters
'features of the moon' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how they can define each other.
'rock at first put in boxes' is the wordplay.
'at first' says to take the initial letters.
'put in' indicates putting letters inside (back).
'boxes' becomes 'crates' (crate is a kind of box).
The first letter of 'rock' is 'r'.
'r' inserted into 'crates' is 'CRATERS'.
Can you help me to learn more?
(Other definitions for craters that I've seen before include "Depressions" , "Shell-holes" .)