Ancient Greek coin (4)
'ancient greek coin' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for obol that I've seen before include "bit of Ancient Greece" , "Old Greek coin" , "Jargon, slang" , "Old bit of lucre" , "Ancient Greek coin, six to the drachma" .)
(Other definitions for stater that I've seen before include "One expressing clearly; ancient Greek coin" , "One saying something" , "making declaration?" , "One specifying" , "Greek currency as earlier" .)