Crossword Genius

Witness, only on kerb, never wholly crossing roundabout? (8)


I believe the answer is:


I'm a little stuck... Click here to teach me more about this clue!

'witness' is the definition.
(I know that onlooker is a type of witness)

'only on kerb never wholly crossing roundabout?' is the wordplay.
I cannot really understand how this works, but
'on' is present in the answer.
'roundabout?' could be 'o' (resembles a roundabout) and 'o' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for onlooker that I've seen before include "Bystander" , "He's not involved - just observing" , "One who watches without becoming involved" , "Spectator" , "Person watching" .)

I've seen this clue in The Times.
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