Unfriendly half of 1A's old agents left; ... (10)

I believe the answer is:

antisocial

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But I can't explain it at all! Could you help me understand?

(Other definitions for antisocial that I've seen before include "withdrawn" , "Disinclined to mix" , "Reluctant to mix" , "Avoiding the company of others" , "Disruptive to the community" .)

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