Current copper to assess, to be precise (8)
I believe the answer is:
accurate
'precise' is the definition.
(I know that precise can be written as accurate)
'current copper to assess' is the wordplay.
'current' becomes 'ac' (alternating current).
'copper' becomes 'cu' (chemical symbol).
'to assess' becomes 'rate' (rating is a kind of assessing).
'ac'+'cu'+'rate'='ACCURATE'
'to be' acts as a link.
(Other definitions for accurate that I've seen before include "strict" , "Correct, free of error" , "Precise and exact (8)" , "Exactly right" , "True!" .)