Spot on copper in a box (8)
I believe the answer is:
accurate
'spot on' is the definition.
(I've seen this before)
'copper in a box' is the wordplay.
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'in' is an insertion indicator.
'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 "Totally correct" , "Precise, correct" , "True!" , "Correct, right" , "Bang on" .)