Truck came first loaded with silver (5)
I believe the answer is:
wagon
'truck' is the definition.
(I've seen this before)
'came first loaded with silver' is the wordplay.
'came first' becomes 'won' (I've seen this before).
'loaded with' is an insertion indicator.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'won' enclosing 'ag' is 'WAGON'.
(Other definitions for wagon that I've seen before include "Cart; rail truck" , "Vehicle (on which abstainers travel?)" , "Trailer, railway truck" , "Freight truck" , "Vehicle for transporting goods" .)