Standard portion or medium selection (4)

I believe the answer is:

norm

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

'standard' is the definition.
(I know that norm is a type of standard)

'portion or medium selection' is the wordplay.
'NORM' is hidden in amongst the letters.
But, I am unsure how this is indicated.

Can you help me to learn more?

(Other definitions for norm that I've seen before include "Mean" , "Standard, criterion" , "Small boy" , "Average level of achievement" , "Required or acceptable standard" .)

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