Peeping Tom (6)
I believe the answer is:
voyeur
'peeping tom' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for voyeur that I've seen before include "One who spies on others for sexual gratification" , "A secret spectator, perhaps for sexual gratification" , "A secret spectator, often in sexual situation" , "One who derives pleasure from watching others" , "One who watches" .)