Old French coin (3)
'old french coin' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sou that I've seen before include "Paltry sum" , "Former French coin of low value" , "Old French monetary unit" , "Old French small coin" , "Little money once" .)
(Other definitions for ecu that I've seen before include "Old European currency unit" , "coin once minted in France" , "Former term for Euro" , "It became the euro" , "European money unit; old French coin" .)