Betray the said group of spies or activists

I believe the answer is:

sell

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'betray' is the definition.
(synonyms)

I cannot understand how the rest of the clue works.

(Other definitions for sell that I've seen before include "Exchange for cash" , "Dispose of for money" , "Peddle" , "Vend, auction off" , "Dispose of, maybe in 23" .)

I've seen this clue in the Evening Standard.

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