Small coin once inadequate for Arab market (3)
I believe the answer is:
sou
'small coin' is the definition.
(historical coin)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(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" .)