One standing armed at the crossroads? (8)
I believe the answer is:
signpost
'one standing armed at the crossroads?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
This is all the clue.
(Other definitions for signpost that I've seen before include "Waymark" , "this will give you directions" , "It shows the way" , "Direction marker" , "Clue" .)