Small numbers a breach of the rules (2-2)
I believe the answer is:
no-no
'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.
Can you help me to learn more?
(Other definitions for no-no that I've seen before include "Non-starter" , "turn-off for some?" , "Something totally unacceptable (informal)" , "Impossibility" , "Unacceptable act" .)