Most cunning copper on trial (6)
I believe the answer is:
cutest
'most cunning' is the definition.
('cute' can be a synonym of 'cunning')
'copper on trial' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'on' is a charade indicator (letters next to each other).
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'cu'+'test'='CUTEST'
(Other definitions for cutest that I've seen before include "Most appealing" , "Most quaint and attractive" , "Most attractive in an endearing way" , "Most clever" , "Most pretty" .)