Crossword Genius

Small numbers a breach of the rules (2-2)

Ross

I believe the answer is:

no-no

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'small numbers' is the definition.
(no is short for number as in no. 1; 'small' indicates a short form)

I don't understand the remainder of the clue.

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(Other definitions for no-no that I've seen before include "Non-starter" , "turn-off for some?" , "Something totally unacceptable (informal)" , "Impossibility" , "Unacceptable act" .)

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