Ancient coin (4)
I believe the answer is:
obol
'ancient coin' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for obol that I've seen before include "Small currency" , "Old Greek coin" , "Jargon, slang" , "Old bit of lucre" , "Ancient Greek coin, six to the drachma" .)