Shell sometimes used as money (6)
I believe the answer is:
cowrie
'shell sometimes used as money' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cowrie that I've seen before include "Mollusc (shell used as money)" , "Shell once used as currency" , "Marine gastropod with a shiny shell once used as money in some parts" , "Brightly marked seashell" , "Shell sometimes used as form of money" .)