Correct copper in a box (8)
I believe the answer is:
accurate
'correct' is the definition.
(associated in meaning)
'copper in a box' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'in' indicates putting letters inside.
'box' becomes 'crate' (crate is a kind of box).
'a'+'crate'='acrate'
'cu' going inside 'acrate' is 'ACCURATE'.
(Other definitions for accurate that I've seen before include "Correct in all details" , "strict" , "Correct, free of error" , "not deviating from the truth?" , "telling it like it is" .)