Best-dressed males standing trial (8)
I believe the answer is:
smartest
'best-dressed' is the definition.
(smart can mean well-dressed)
'males standing trial' is the wordplay.
'males' becomes 'rams' (male sheep).
'standing' says the letters should be written in reverse.
'trial' becomes 'test' (synonyms).
'rams' back-to-front is 'smar'.
'smar'+'test'='SMARTEST'
(Other definitions for smartest that I've seen before include "Extremely bright" , "Most wise" , "Cleverest; most chic" , "Most clever" , "Brightest or best dressed" .)