Copper on trial is most shrewd (6)
I believe the answer is:
cutest
'most shrewd' is the definition.
Both the answer and definition are superlative adjectives. Perhaps they are linked in a way I don't understand?
'copper on trial' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'trial' becomes 'test' (synonyms).
'cu'+'test'='CUTEST'
'is' is the link.
Can you help me to learn more?
(Other definitions for cutest that I've seen before include "Most clever" , "Most quaint and attractive" , "Most astute" , "Most sweet and endearing" , "Smartest" .)