Onlooker’s cheers welcomed by policeman dropping in (9)

I believe the answer is:

spectator

'onlooker's' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for spectator that I've seen before include "Watcher" , "Onlooker; magazine" , "Person watching" , "Cat poster (anag.)" , "observer" .)

I've seen this clue in The Times.

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