Spies, you are lights in state, but of base stuff
Who, when you've burn yourselves down to the snuff
Stink and are thrown away. End fair enough
Meaning well-conditioned or having a pleasing form, but also meaning serving the government nobly. Whichever meaning you take it as, the “base stuff” following the phrase provides a contrast, meaning that one has to be of low moral character to be an effective spy.
On Spies