Box in which earth is found (6)
I believe the answer is:
create
'is found' is the definition.
(to found is to establish or create)
'box in which earth' is the wordplay.
'box' becomes 'crate' (crate is a kind of box).
'in which' indicates putting letters inside.
'earth' becomes 'E'.
'crate' enclosing 'e' is 'CREATE'.
(Other definitions for create that I've seen before include "Make, Porduce" , "Establish" , "make a scene" , "Design" , "Build" .)