Smartest copper brought to trial (6)
I believe the answer is:
cutest
'smartest' is the definition.
(I've seen this before)
'copper brought to trial' is the wordplay.
'copper' becomes 'cu' (Cu is the chemical symbol for copper).
'brought to' means one lot of letters go next to another.
'trial' becomes 'test' (synonyms).
'cu'+'test'='CUTEST'
(Other definitions for cutest that I've seen before include "Most astute" , "Most appealing" , "Most clever" , "Most pretty" , "Most sweet and endearing" .)