What’s sickening to numbers of Romans (5)

I believe the answer is:

toxic

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'what's sickening' is the definition.
I can't judge whether this defines the answer.

'to numbers of romans' is the wordplay.
'numbers of romans' becomes 'xic' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to'+'xic'='TOXIC'

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(Other definitions for toxic that I've seen before include "Harmful - cox it (anag)" , "Poisonous, noxious" , "word describing bad things" , "Deadly - venomous" .)

I've seen this clue in The Times.

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