Copper trial is the most engaging (6)
I believe the answer is:
cutest
'copper trial is the most engaging' is the definition.
I can't tell whether this definition defines the answer.
'copper trial is the most' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'trial' becomes 'test' (synonyms).
'is the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'most' means to remove the last letter.
'cu'+'test'+'t'='cutestt'
'cutestt' with its last letter removed is 'CUTEST'.
Can you help me to learn more?
(Other definitions for cutest that I've seen before include "Most attractive in an endearing way" , "Most clever" , "Most astute" , "Most appealing" , "Most quaint and attractive" .)