Correct copper in a box (8)
I believe the answer is:
accurate
'correct' is the definition.
(similar in meaning)
'copper in a box' is the wordplay.
'copper' becomes 'cu' (Cu is the chemical symbol for copper).
'in' means one lot of letters goes inside another.
'box' becomes 'crate' (crate is a kind of box).
'a'+'crate'='acrate'
'cu' placed into '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" .)