Nosy parker (6)
I believe the answer is:
voyeur
'nosy parker' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for voyeur that I've seen before include "One who derives pleasure from watching others" , "A secret spectator or peeping Tom" , "A secret spectator, perhaps for sexual gratification" , "A secret spectator, often in sexual situation" , "Naughty observer" .)