Found sapphire at last in box (6)
I believe the answer is:
create
'found' is the definition.
(to found is to establish or create)
'sapphire at last in box' is the wordplay.
'at last' says to take the final letters.
'in' means one lot of letters goes inside another.
'box' becomes 'crate' (crate is a kind of box).
The last letter of 'sapphire' is 'e'.
'e' inserted into 'crate' is 'CREATE'.
(Other definitions for create that I've seen before include "Devise" , "Come up with" , "found" , "Make, Porduce" , "Bring into existence" .)