Obsolete gold coin (6)
I believe the answer is:
guinea
'obsolete gold coin' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for guinea that I've seen before include "Old coin, twenty-one shillings" , "Obsolete gold coin" , "Old sum of money - west African republic" , "so-called pig that's experimentally tested?" , "Former British coin worth 21 shillings" .)