Spectator (6)
'spectator' is the definition.
(I know that onlooker is a type of spectator)
This is all the clue.
(Other definitions for onlooker that I've seen before include "Spectator" , "One who watches without becoming involved" , "One observes" , "Person watching" , "He's not involved - just observing" .)
'spectator' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for observer that I've seen before include "Non-participant; a paper" , "He sees" , "'Onlooker, spectator (8)'" , "One who watches" , "monitor" .)
'spectator' is the definition.
(I know that spectator can be written as viewer)
This is all the clue.
(Other definitions for viewer that I've seen before include "Review for TV watcher" , "Spectator" , "One seeing" , "Observer" , "One watching, eg, TV" .)