Witnesses using toilet work back at Queens (9)

I believe the answer is:

onlookers

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'witnesses' is the definition.
(I know this)

'at queens' is the wordplay.
'at' becomes 'on' (location).
'queens' becomes 'lookers' (I can't explain this - if you can you should believe this answer much more).
'on'+'lookers'='ONLOOKERS'

'using toilet work back' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for onlookers that I've seen before include "Bystanders, rubber-neckers" , "Spectators" , "They watch" .)

I've seen this clue in the Sydney Morning Herald.

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