Mimic boy dressed in Roman style (12)
I believe the answer is:
'mimic' is the definition.
(I know that mimic can be written as impersonator)
'boy dressed in roman style' is the wordplay.
'boy' becomes 'son' (synonyms).
'dressed in' indicates putting letters inside (inserted letters are dressed in others).
'roman style' becomes 'imperator' (Roman title or style).
'son' inserted within 'imperator' is 'IMPERSONATOR'.
(Other definitions for impersonator that I've seen before include "he takes off" , "ape" , "One's often acting" , "Impressionist" , "Mimic" .)