Rev, you dreadful Peeping Tom (6)
I believe the answer is:
voyeur
'peeping tom' is the definition.
(synonyms)
'rev you dreadful' is the wordplay.
'dreadful' indicates anagramming the letters.
'rev'+'you'='revyou'
'revyou' is an anagram of 'VOYEUR'.
(Other definitions for voyeur that I've seen before include "One taking pleasure in watching others" , "Person who watches others in secret" , "One views for titillation" , "Secret or furtive sort of spectator" , "One who derives pleasure from watching others" .)