Before trial, butters up the best-dressed (8)
I believe the answer is:
smartest
'best-dressed' is the definition.
(smart can mean well-dressed)
'before trial butters up' is the wordplay.
'before' means one lot of letters go next to another.
'trial' becomes 'test' (synonyms).
'butters' becomes 'rams' (I can't justify this - if you can you should believe this answer much more).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'rams' back-to-front is 'smar'.
'test' put after 'smar' is 'SMARTEST'.
'the' is the link.
Can you help me to learn more?
(Other definitions for smartest that I've seen before include "Most wise" , "Cleverest; most chic" , "Most intelligent or most stylish" , "Extremely bright" , "Most clever" .)