Most charming copper on trial (6)
I believe the answer is:
cutest
'most charming' is the definition.
'copper on trial' is the wordplay.
'copper' becomes 'cu' (Cu is the chemical symbol for copper).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'trial' becomes 'test' (synonyms).
'cu'+'test'='CUTEST'
(Other definitions for cutest that I've seen before include "Most sweet and endearing" , "Most pretty" , "Most astute" , "Most appealing" , "Smartest" .)