Bullion unit (5)
I believe the answer is:
ingot
'bullion unit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ingot that I've seen before include "Unwrought metal cast in a mould" , "Pig" , "A brick shaped lump of gold" , "Mass of unwrought gold, for instance" , "Mass of metal, especially gold or silver" .)