Resist Sergeant Major's demand (4)

I believe the answer is:

shun

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'resist' is the definition.
The answer and definition can be both to do with social activities as well as being verbs in their base form.
Maybe they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for shun that I've seen before include "Ostracise" , "Keep away from, have nothint to do with" , "Cold-shoulder" , "Persistently reject" , "Deliberately have no contact with" .)

I've seen this clue in The Guardian.

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