Small coin once inadequate for Arab market (3)

I believe the answer is:

sou

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'small coin' is the definition.
(historical coin)

I cannot understand how the remainder of the clue works.

(Other definitions for sou that I've seen before include "Something of little worth" , "Defunct French coin" , "Very little to spend" , "Former French coin of low value" , "Paltry sum" .)

I've seen this clue in The Times.

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