Par (4)
(Other definitions for usual that I've seen before include "Customary - customary drink" , "not rum?" , "Stock" , "Standard" , "Normal, habitual" .)
'par' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for norm that I've seen before include "Typical pattern or situation" , "Benchmark" , "(The) accepted way" , "Bacterium, virus, etc" , "Required or acceptable standard" .)