Fort Knox supply (4)
I believe the answer is:
gold
'fort knox supply' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gold that I've seen before include "One of the Magi's gifts" , "Colour of archery target's centre" , "Thin sheet of precious metal" , "bull's-eye" , "Element (Au)" .)