Voyeur on trial, dreadfully subversive (13)
I believe the answer is:
revolutionary
'subversive' is the definition.
('revolutionary' can be a synonym of 'subversive')
'voyeur on trial dreadfully' is the wordplay.
'dreadfully' is an anagram indicator.
'voyeur'+'on'+'trial'='voyeurontrial'
'voyeurontrial' with letters rearranged gives 'REVOLUTIONARY'.
(Other definitions for revolutionary that I've seen before include "New" , "Innovative" , "French calendar?" , "Insurgent - involving dramatic change" , "agitator" .)